Submarines can't function under threat of undeterred ASW patrols...
The Hitlerites lost the Battle of the Atlantic because their submarine forces were, as submarines are now, heavily dependent on naval intelligence organs to ambush warships with extremely short range weapons, and heavily vulnerable to air attack during the intrusion phase, which was easily disrupted by aircraft carriers and hunter-killer formations. The entire lesson of WW2 is that submarines need air cover to attack and destroy naval forces when the enemy naval forces have any appreciable ASW. The Russians recognize this, as does the PRC, which is why the PRC is pushing out into the SCS with sandbar islands, and why the Russians are whinging about Kuznetsov being in drydock for the next 500 years because the Northern Joint Command needs sea-based air cover to properly protect the bastion at range over the Arctic Ocean.
The only people who never fully appreciated the utility of ASW capabilities were the Japanese,and they were a somewhat special case.
The UK has barely four operational SSNs at any one time, so I don't think either Russia or PRC require much ASW to handle that threat. What they have is more than adequate for the most part...
...except for the fact that the UK has a pair of supercarriers with fighters that can effectively neutralize any significant air defense response to allow the MPAs to fly. A CVF protected by two Type 45s and a few Type 23s is a lethal threat that is unlikely to be assailable by anything short of one of three Backfire regiments (the one at Olenya) using nuclear missiles. A carrier in any form presents a strategic threat, one that has to be handled immediately, and is often heavily defended against all forms of attack both undersea and airborne.
This is not surprising. A carrier battlegroup is often five or six times as many ships as a single submarine. Of course it's more survivable.
A submarine by itself (and they are always by themselves, because wolfpack tactics are dead and buried, thus submarine threats are arguably less dangerous than they used to be) is mostly an annoyance unless it packs nuclear weapons, then it is also a strategic threat. Of course, Astute has no nuclear weapons, either now or for the foreseeable future, so it's mostly a threat to strategic forces, but not a strategic threat in of itself. It's fairly easily handled by picket forces and ASW aircraft.
The only thing Astute does that is noteworthy in a big war is penetrate bastions and attack portions of the Russian strategic deterrent, but there aren't really enough Astutes for the UK to do anything substantial without incurring the wrath of either the Russian Navy or the Strategic Rocket Forces were they to attempt to decapitate Russia's nuclear arsenal, so it's a bit useless at the moment by itself.
There would need to be an Astute for every operational Russian SSBN, so about eight ready, which means maybe a dozen or so SSNs. More once you account for immediate losses from MPAs and anti-submarine screens, though. So maybe half again or double that number. So very nearly, or more than, twenty SSNs would be required. The UK clearly cannot afford this, even if it scrapped CVF, but it would be about equal the number of USN submarines assigned
The existence of CVF lets the UK stretch its limited resources more by having the ability to neutralize MPAs and allow its smaller-than-adequate quantity of SSNs survive long enough to potentially liquidate more SSBNs, by shooting down Tu-142s and IL-38s that would come running to the rescue of the bastion, and maybe killing the SSBNs fast enough that they can only partially fire their missiles. Maybe. At the very least the CVF would allow SSNs to survive the intrusion phase of penetrating the Russian SOSUS line into the bastion without incurring casualties by neutralizing picket frigates, ASW helicopters, and curious MPAs.
Since Russia has no operational flattops anymore for a bit there's no real, serious method of deploying frontal aviation forces to the Arctic or North Sea, which makes the bastion vulnerable to direct incursion by a submarine-CVBG task force. Prior to Kuznetsov's unfortunate welding accident there would have been something of a battle between the CVs and potentially this could have inflicted adequate losses to force the CV to retreat, which would deny the enemy SSNs their air cover, and make them easily killed by MPAs. This no longer being the case is rather significant since it makes the Borei/Delta SSBN force extremely vulnerable to ASW action in any form.
That would be the only practical purpose of the Astute-CVF team in a "big war", or any submarine force, really: neutralizing SSBN forces. There is no other purpose for a navy in a war between nuclear armed nations besides this because lessening the damage from strategic bombardment forces is all that can be done by forces which require massive, fixed-installations to function.
The UK would need to invest in ABM protection of its military harbors to go further in regard to making the RN more important in a "big war". Since it hasn't, the RN's sole actual purpose is fighting "small wars" so carriers are no more a "political" (what does this mean?) move than a recognition that the UK is neither sufficiently economically developed nor moral-politically motivated enough to fight and win a nuclear conflict. Not even the CPSU was.
To make the RN relevant in a "big war" requires hardening harbors against nuclear attack through ability to intercept and destroy incoming nuclear warheads. Otherwise, it is useless outside of the first opening few days where it will likely be used (and expended) in dealing damage to the ballistic missile forces before they launch in order to reduce damage to the homeland. Until that becomes a economic possibility it's a bit useless to discuss what can be done in "big war" but it's fine to discuss what the RN can do in the next Sierra Leone or Iraq War.
The answer currently is "quite a lot". Certainly more than they did in 2003, which itself was rather impressive with the capture of the Al-Faw Peninsula. The UK could likely do Al-Faw again, all by itself this time, with the force structure of 2022 tbh.
In other words, what you consider to be a marginally effective force at best, incapable of fighting a "big war", would be recognized by Gorshkov and the Soviet Navy as being an exceptionally dangerous scalpel explicitly built to penetrate and destroy the future SSBN (Borei) within its own bastion. Perhaps this has some truth, after all CVF has been in development for a very long time and JSF was ultimately descended from 1980's future strike aircraft, which were going to be built to strike Murmansk with nuclear weapons. Naturally, this would be with nary a scratch able to be made by Soviet-era equipment between the stealth/low observable design of the Type 31/Type 45/CVF ships, the F-35B JSF's doubling of combat radius over VAL equipped nuclear strike carrier Midway, the Astute's LFA sonar stealth shaping, and the exceptional lethality of the Spearfish torpedo against deep diving Boreis making it unlikely that countermeasures would work or that existing torpedo weapons could attain range superiority even if a TMA solution were found before the British. Which itself is highly unlikely in any case, as the British have one of, if not the, best trained navies in the world.
If you take away the CVF then the lethality of Astute drops to extremely middling quantities, at best, against a defended, both-eyes-open bastion, even without the Kuznetsov. Certainly not enough to seriously intrude on the bastion's ability to deliver its nuclear missiles to target. Between casualties caused during the intrusion phase by Astutes unprotected against Naval Aviation, the subsequent inability to escape or ward off attackers without giving away their position through engagement, and the lack of naval intelligence provided by a on-call JSF wing with advanced SAR radars for naval search and detection of hostile fleet units, the Astutes are going in blind, unprotected, and against an awake predator (MPA).
Arguably the only thing the UK is lacking at the moment is a SURTASS-style ship, but they're not planning to fight in the Pacific where deep ocean battles are the supposed norm, so that's fine.
It's kind of funny actually, but I suppose it's one of those "other side of the hill" moments. The Royal Navy's current force structure and ship designs are fine. The "only" problems are that the UK lacks hull numbers, a few important support ships, like fast replenishment ships and repair dock ships, and a proper AEW for its carrier. It could use another CVF, about five to seven more SSNs, and another four to six Type 45s, on top of a triplet of fast supply ships, some submersible repair dock ships with the barge (I forgot the name), twice as many P-8s, another squadron of JSFs, and a proper AEW like a V-22 or something, but none of that will be happening, so any significant investment in new or particularly more capable ships than what exists wouldn't happen either.
All these problems are ultimately economic in nature and thus somewhat intractable for the UK to solve. Since it is a primary beneficiary of the current economic world order it would also be unlikely to want to change this, too, because change means the UK's position could end up worse than it already is. The PRC is rather different, but it also has fairly cheap agricultural laborers to draw on, as Britain and America did in the 19th and 20th centuries respectively.
Also using Sweden and Germany as examples of "better placed" militaries is...interesting, to say the least. I somehow don't think that spending the most per capita on defense and having a military with the funding of the UK's yet still incapable of fielding more than a mechanized battalion or a single fighter squadron, at best, is evidence against British policymaking. The UK has simply been handed a bad lot of suffering from a Soviet-style economic contraction in the middle of the last century, going from preeminent world power to a first-rate regional power, and declining, yet it's made do for itself quite well.
It could very well have ended up worse, and it likely will become worse as this century drags on, with the only question being whether the UK can become worse slower than its enemies. The answer seems to be yes, at least so far. No one else in Europe could have built CVF. No one else in the world could have maintained the Cold War standard of training of fighter pilots, or actually improved their ability to fight wars from the sea, as judged by the fact that only the UK has done this from 2003 to 2022. It may have sacrificed its mechanized ground troops to an ill-fated adventure trying to fund Spain's much-desired ASCOD 2, but this is not particularly relevant, as mechanized ground troops don't seem to be very important in all seriousness.
Britain is an island. It doesn't need an army. It needs an air force first and a navy second.
