I know Zen wants this to be a positive thread focusing on the positive reasons for selecting a project and what it might have achieved but I feel that both the Thin Wing Javelin and the SR.177 have flaws that probably should dictate them being cast aside.
Gloster was not really equipped to design modern jet fighters, the Javelin took eight years to develop (it only served the RAF for 12 years!) and was beset by aerodynamic problems but was a serviceable enough aircraft when the bugs were ironed out but it took penny packets of various models to get it right. Thin Wing Javelin started in 1953, three years before the Javelin entered service and it grew in size and weight and hopes of having it in 1958 evaporated as redesign followed redesign. TWJ was still barely supersonic (F.153 with 20,550lbf Ol.7R Mach 1.4 at 66,000ft on full reheat, later P.376 with 28,500lbf Ol.21 Mach 1.66 at 54,000ft) trading altitude for speed. The F-101 had comparable AUW but could achieve Mach1.6 but only 51,000ft absolute ceiling. It must be emphasised that F.153D was an interim design, TWJ was a stand-in to provide head-on intercept capability until the F.155 was ready so it wasn't indispensable. The CF-105 was a fresh-sheet design and looked an excellent package but it was later than F.153 and overlapped F.155 so there seemed little point in buying CF.105 (also the Dollar issues). I'd rather funnel the funds into F.155
The SR.177 is an unknown, Saro had no fighter design track record and their five-year development programme seemed very optimistic. The SR.177 was a niche interceptor, much like the Lightning. Its hard to pinpoint any advantage it had over the Lightning for the RAF. For the Royal Navy I have never really seen why the Admiralty joined OR.337. I'm not sure there ever was a high-altitude threat to the carrier groups that Sea Slug couldn't handle. Also, operating HTP-fuelled rockets aboard carriers would have been problematic. It lacked enough multi-role capability, loiter range and weapons capability; 2x Red Top and 2x underwing hardpoints for bombs was pretty light compared to comparable USN fighters. I have a suspicion that the Admiralty tagged along in the realisation buddying with the RAF was their only chance of getting a new fighter given the uneconomic proposition of arming the small FAA fighter force with a new design and the SR.177 was the only suitable candidate that could operate from a carrier.
As pointed out in another thread a supersonic Scimitar (Type 576) was probably a redesign too far for that airframe, but I think a fresh-sheet design of similar size and capability (4-6x hardpoints, AI.23, 50,000ft target interception, Mach 1.8, 50,000lb AUW) would have been the best approach for the Navy. Of course the money wasn't there for that.