The basic 12.7 x 81 Vickers round would have increased effective
range to at least 800 yards; the powerful 12.7 x 120 V.669 version with a muzzle velocity of some 3,000 fps would easily have reached 1,500 yards in air combat.

Apparently the 50-caliber Vickers was considered by the RAF
during the war:

http://www.quarry.nildram.co.uk/Vickers.html