I have yet to see a clear explanation why the British failed to produce a 120 mm destroyer gun mount capable of high angle fire prior to WW2... when they had introduced the Mark VIII version already from the mid 1920s and planned its use on the A class leaders as well.
To enable the guns to elevate high-enough, the trunnions would have to be placed much higher. This means that in a low angle role, loaders would have to reach above their heads to load shells into the breech.
The 40 degree elevation of prewar mountings was certainly sufficient to contribute to the defence of other units from air attack, and prior to the war Britain did not think that destroyers were valuable enough to be attacked from the air.
Logically they should had just updated Mark VIII to use separate ammunition instead of fixed ammunition to increase rate of fire. The standard gun should had been the 4.7"/45 Mark F Vickers built for Spain only with separate ammunition. Instead they tried to design a 5.1 in gun with fixed ammunition, when they already found the 4.7in fixed round too heavy, rejected that on grounds of the fixed round being heavy (well duh I say, did you really need to build it first to get it?) and then went and developed yet another round in 4.5in... again with fixed ammunition heavier than the one they found problematic on Nelson. It's not making any logical sense.
So just point out the obvious...
The Spanish 4.7 inch gun and the MkVIII fired shells lighter than the 55lb shell fired by the Mk IX, let alone the 62pdr shell fired by the Mk XI. Not to mention that export designs tended not to meet UK requirements.
The Mk VIII was not intended to be fitted to Codrington. Codrington was originally planned to have a single Mk IX gun on a mounting capable of 60 degree elevation in B position, which is where the confusion may have crept in.
As for the fixed-ammunition 4.5in guns, they were initially only used by large capital ships in between deck mounts. Shells were moved in hoists to handing rooms directly around the mountings, and only had to be moved a short distance. Such ships are considerably steadier than destroyers, meaning it was much easier to handle the shells, and the between-deck design meant that the shell hoists were likely much closer to the mountings than 4.7" Mk VIIIs.
These mountings were still criticised for their low rate of fire when placed on the Battle class.
My preference would something similar in concept to the 4.6in "bastard" mentioned in Nelson to Vanguard, only with enough room for recoil, essentially putting the 62pdr 4.7"/50 Mk XI in a new mounting with an 80 degree elevation. You get the excellent low-angle performance of the 4.7"/50 Mk XI with its 62pdr shell, with the high-angle performance of the 4.5 inch mountings. The mounting should be a "long stalk" design, with the hoists rotating with the gun house, or alternatively, an upper-deck design similar to the 4.5" Mk VI with a gunbay immediately below the mounting, in which ammunition is transferred to hoists that do rotate with the mounting.
A large destroyer, similar to L.72, L.90 or a Daring could be built at yards with slips large enough to take them, with a smaller destroyer, similar to the War Emergency types, Weapon class or Gallant class being built at yards with shorter slips.
