I certainly don't want to rain on a whiff-parade, but here are some interesting things to think about:
4. With the existence of a homogenous class of modern, combat proven battleships that are still capable makes retention, upgrading and modernisation a no brainer.
There were plenty of modern, combat proven British cruisers and aircraft carriers that didn't make it past the mid-1950s, so those qualities in themselves were no guarantee of survival. Basically there were two fundamental problems:
1. Britain was broke. We couldn't afford to maintain large numbers of ships, let alone re-build them. Also, these things need a cast of thousands to run them: in wartime you can conscript, but in peacetime you have to pay good enough wages to compete with cushy civilian jobs, in a world that's had it up to
here with war. Conscription did continue after the war, but even early on, it was obvious that politically, that was only going to be tolerated for so long.
2. A lot of pre-war and war-built ships that appeared superficially intact were actually quietly shagged, partly due to poor (rushed) construction and partly due to heavy and brutal usage. Quite a few modernisations (Tiger, Blake, various destroyers) and conversions (Victorious, various destroyers) were actually tried, but most of them ran into horrendous problems, cost increases and delays. Often, the problems wern't with the "sexy" stuff like weapons and radars, but with the mundane-but-fundamental stuff like electrical systems, steam plumbing and structures. it's worth reading the relevent chapters of D.K.Brown's excellent
Rebuilding the Royal Navy for an insider's view of the conversion/upgrade business, many if most of which seem not to have been worthwhile in hindsight.
A few other relevent issues:
If the 6000 ton Counties could take Seaslug then the 8,000 to 10,000 ton 6" cruisers certainly could, so it wasn't a case of the latter being too small, rather it was that they were thought to be too worn out for it to be worth spending the money on them. The other problem was the sheer scale of the conversion: a complete Seaslug installation is a completely different shape to a system of gun turrets, being essentially a long, horizontal hangar with a launcher at the end of it. fitting such a system into any gun ship would entail a massive and fundamental re-build, far in excess of "merely" changing turrets or gun calibres. Had the RN gone for either the US Terrier system (which we asked to be developed in the first place) or something similar with vertical stowage, then the problem might well have been easier to solve.
A battleship's "unique selling point" is it's big guns, so any scheme that involves removing them risks turning it into just a big, expensive generic hull: this is the logic that killed all the proposals for extensive
Iowa conversions. However, if you're going too keep big guns, then you're going to keep all the problems that come with them, such as muzzle blast, which is particularly hard on relatively flimsy structures like missile launchers and radars. This severely constrains the extent to which you can refit modern systems to a big gun warship.
Having said all this
if you must have a modernised KGV, then this is how I'd do it:
1. Remove B-turret and replace it with better accomodation and command spaces. The loss of 2 guns from the broadside will be less relevent to post-war fire-support missions than it would be to ship-to-ship combat, and this turret's muzzle blast is a high threat to other systems. It also saves you a load of crewmen too.
2. Replace P1, P4, S1 and S4 turrets with either twin 4.5" Mk.6 or twin 3" Mk.6.
3. Replace P2, P3, S2 and S3 turrets with twin Mk.11 Terrier launchers. This saves a load of manpower too.
4. Add a new superstructure block that runs across the ship in the former aircraft deck space. This provides more internal volume for modern systems and carries Terrier directors on it's roof to either side.
5. Remove most of the light flak. This saves yet more manpower.
The result is a ship that can bombard a coast in support of an amphibious operation while providing a degree of SAM protection (or
perceived SAM protection) for itself and the amphibious ships around it.