IIRC The lack of 50's had something to do with tests in ~1930 which resulted in the adoption of the Browning. The same tests showed no advantage of the 50 vs 303. This became doctrine and nobody really considered the a/c changes from fabric to metal skins with some armour.
When the problems with this approach became obvious in the late '30's there was worry about 50 proof armour so the decision was made to go straight for 20mm. Unfortunately the 20mm was not well developed and the prevvious applications were mostly engine mounted. This combined with general stress on engineering resouurces with rearmanent meant the development fell though the cracks until the need became critical. It didn't take too much time to fix the guns but a lot longer to arrange for 4x20 in the Spit wing.
The problem with the turreted 50's and 20mm was basically just the fact the turrets had to be redesigned, developed and debugged. This was a much bigger job and hence took longer.
After the war the pendulum went a bit far in bigger guns eg the 5 inch (4.5 ?/5.4?) recoiless automatic cannon. This chewed a lot of resources before it was realised the engine development wasn't going fast enough to deal with a weight of about a ton without ammo.