I'm afraid the information about the Mk VII .303 round is incorrect. I don't have access to any referance books at the moment but the Mk VII was a spire pointed round. It superceeded/ was introduced to replace, the Mk VI which was round nosed and parellel sided in 1910 ! Basically the pointed tip gives better supersonic performance and reduces drag at sub sonic speeds. Basically better retention of muzzel velicity, giving a flatter trajectory, less aim-off and a greater terminal energy transfer.
The Mk VII z used a nitrocellulose based propellant and this would have been virtually universal by the time of the Battle. Manufacturers coloured the primer annules, black prior to 1918 and after purple for Ball, red tracer, explosive orange and incendiary blue. Apparantly to ease identification bullet tips were also painted, initially this would have been done by service armourers so the blue colour is not necessarilly incorrect, local variations being known. However Blue tip - incendiary, white air to air short range day tracer, gray air to air short range night tracer, black - observation (flash on impact), no colour ball.
Explosive/inceniary rounds were used, some being derived from the Buckingham designs of the First World War, however they tended not to be terribly effective, bear in mind that the bullet diameter is only .303 (7.7 mm)There simply isn't a lot of free payload in these calibers. Also the vast majority of rounds would have passed straight through a wing or fuselage section, very likely the exit hole being larger as the round would have been upset and accompanied by high speed fragments from the entry hole. But certainly not fist sized. (viz. Fabric patches on Hurricanes and I believe that one of the Memorial Flight Spits still has battle damage repaired wings, not BoB though ?)
The Mk VII was in service, I have dug then out of the sand at Dunkirk with mfg date stamps in the mid 36s.
To improve performance you need to go up to 20 mm or so, as the german cannon and later marks of the Spit and Hurricane. Slower rate of fire, worse trajectory, slower muzzel velocity so harder to deflection shoot. If anything it would have been the mechinical computing gunsights coming into service that would have improved the average pilots lot as these would at least allow accurate shooting of cannon.
Interestingly enough the ADEN cannon series is a reworking of the German MG 151/15 and MG 20, both of which the Luftwaffe would have used in the BoB.