What if the UK opted to go for nuclear propulsion for its CVF/QE class carriers?
It's going to increase costs quite a bit. Design and construction will be more expensive, whilst you might make some savings on oil and tankers that will be offset by increased maintenance costs, and decommissioning will be significantly more involved and costly. There will also be increased personnel costs as nuclear propulsion is a much more specialised field, and you can bet that the civilian nuclear industry will try poaching people as soon as their minimum terms of enlistment are met. Throw in some new port facilities as well. The Admirals essentially traded away a large numbers of smaller ships to gain the two
Queen Elizabeth-class aircraft carriers, I'm not sure the extra money would be there for nuclear propulsion without cutting the Fleet to such a shadow of itself that the proposals would be laughable.
Another challenge is that once the ships are built there's going to be a large gap of possibly a decade or more until they need to go back in for refuelling and overhaul, that means you're going to have a hard time maintaining skills and knowledge. See the issues that have come up with nuclear submarine production. The US Navy with their ten nuclear aircraft carriers means they can launch one every few years and by the time they get towards the end of the line it's time to start carrying out Refueling Complex Overhaul (RCOH) programs on the earlier ships.
That does raise another question – if the Royal Navy acquired nuclear power aircraft carriers what happens when they need to undergo refuelling? The UK is never going to be able to have more than one facility so it will have to do it one after he other, assuming of course that they get two. Since the Americans, who have much more experience, can apparently take nearly three years to carry it out that means a potential five to six year gap where the UK will only have one active aircraft carrier. Even then they will still require time in port for regular maintenance so there would be periods where you might have no aircraft carriers available.
Could that really be justified against the larger costs involved? I'm not sure you could get away with it even if you built three aircraft carriers since IIRC the rule of thumb is three are needed to guarantee at least one is always available. The only way I can see it working is if the UK decided to forgo refuelling them and build new replacements after twenty-five years. That's going to mean larger costs spread across a smaller timeframe making them comparatively more expensive than American carriers.