they would have been better off making Vanguard a 3 turret ship, and taking the guns and turrets from a Nelson class battleship.
Actually Vanguard was 4 turret version of Lion.
In early 1939 after examination of production capacity it was clear that limiting factor of starting new battleship is production limit of 16” turrets.
It was possible to build 7 such a turrets per year. So it was possible to start 2 battleships with 3 turrets per year and 3 battleships every 3 years.
After some calculations of possible building new ships in most probable enemies ( that time Germany and Japan ) it was clear that in somewhere around 42-43, Britain will be in short of battleships.
Upgrading of industry was considered, and soon approved to increase production capacity to start 3 battleships per year, but it will take time. It was impossible to have extra new battleship in 1943 ( over first 2 Lions ) If she had to be armed by turrets from upgraded industry.
But it was possible to build one extra battleship if extra guns and turrets were to be obtained some way.
So it was start conception of utilizing existing turrets with 15” guns.
Problem was not guns – but turrets. Taking guns from Nelsons changed nothing, as it was need to produce new turrets. Taking whole artillery from Nelsons was not even considered, as those battleships were needed to be on duty, and were not considered to be scrapped.
Expected function of Vanguard was closer to battlecruiser than to battleship. Battlecruiser in more “modern” post Jutland sense of the word. She was considered to be used in far east to fight various cruisers, Kongos, new 12” gunned large japan cruisers ( British believed that they were constructed, we know now that it was myth ) and so one.
No classical fight against first class new battleships as a first function, but possible if really needed.
She had to “keep the sea” until full scale task force arrives.
That was prewar conception.
After war started everything was shot down. Than conception was resurrected. For some time it was possible to finish her in 1944, but soon other thing had higher priorities and delays made finishing her in time of was impossible.
Finishing her was good or not? From 20:20 hindsight – clearly wrong. But in time when she was started thing seem to be much differend.
And after all – King needed some Yacht