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The British didn't have a suitable HMG in the late 30s or 1940, perhaps they could have.
The .5in Vickers was large, heavy and suffered from jams even though it rarely actually broke. (How do you define reliability?) it also didn't have a particularly high rate of fire.
Provision of truly effective ammo was a problem, No HE rounds and good incendiaries only came later?
Of course the Germans didn't have a heavy machine gun either in 1939/40. They were working on one but it doesn't really see service until 1941.
There was several British (mostly Vickers-made) HMGs, reliable and all that jazz, that required just a will to adopt to RAF needs by 1930s. link
Of course, both British and Germans can buy licence from USA or Italy in mid-30s.
Both the Germans and the British advocates in these retrospective exercises have to be careful not to fall into the trap the US did historically. Over arming the planes for the engines that existed at the time.
The larger, heavier FW 190 airframe may not have worked very well with a 1940 DB 601 engine
Airframe (fuselage, wing, undercarriage, tail, controls & control surfaces) of the Fw 190V1 prototype weighted 760 kg (1765 lbs). Powerplant (engine, prop, cooling cotrols, oil system, cowling) represented around half of in-service Fw 190A's empty equipped weight - 1661 kg (3670 lbs).
Airframe of lightest P-40 ('no letter') went to 2200+ lbs.
Same with adding a cannon to early JU 87s. Even with an early Jumo 211 do you want to try sticking a cannon through the prop? and to what purpose?
An MG FF with a 55/60 round drum is certainly not a tank killer. Perhaps you could use that whacking big gun that used 20 X 138 ammo but it weighed 64kg without ammo.
Yes, I want that big whacking gun (roughly size of Hispano II), with 100 rd box, to kill some 'soft' targets - trucks, carriages, artillery. Ju 87 was no slouch in carrying ordnance, even with the earliest Jumo 211.