The RAF had warehouses full of .303, and whatever reason(s) got considered (rate of fire, density of pattern, etc) that had to be Hewge.
At times, but by 1942/43 the British were using mostly AP and incendiary bullets and had stopped using the Infantry bullets. Even during the BoB only about 37% of the ammo was "infantry" ammo.
While ground MGs used tracer it was not in quantities that Aircraft MGs did. Likewise AP bullets and early incendiary were pretty much special use ammo.
For the US in WW II there were 3 classes of .30 cal ammo.
1, Aircraft Machinegun
2, Ground Machinegun
3, Rifle ammo.
That was in descending order of quality. Nothing to do with accuracy. It was the quality of the brass casings.
If you have a head case separation in an aircraft gun the gun is out of action until the plane lands and an armorer can deal with it.
If you have a head case separation in an ground machine gun the gun is out of action until the crew (covered by other guns and rifle men) can open the gun up, use the broken shell extractor (or change barrels if one is available) and return the gun to action.
If you have a head case separation in a rifle nobody (except the rifleman) cares. A 1919 Browning is putting out 10-20 times the amount of rounds than the best riflemen in the company can do. They have to keep the MGs in action. Rifleman can often pick up a rifle from a casualty.
Using 12-20 year old ammo in aircraft guns was not a good idea (no matter what the treasury men thought)
As mentioned in the video they was also a cost (both money and lost production) in retooling the ammo factories.
A reason the Germans went to the 7.9mm Kurtz round for the MP 44. They could use
some of the same tooling used for the 7.9 X 57 ammo.