It wasn't just for the benefit of the fighter jocks.
Think about all those .303 Browning MGs on all those bomber. Then, also remember that the fighters were primarily operated in daylight and the bombers at night.
In daylight, excess muzzle-flash is "interesting"; at night it can be a whole lot worse than "annoying". Nothing yells "here I am" like cutting loose with the quad .303 Brownings in a Lancaster tail turret with Cordite ammo, which is why it was NOT done. But, never forget one of the key points in "Murphy's Laws of Combat Operations": "Tracer works both ways". With the aerial MG belts being filled according to the instantly mensurable sequence, "TITS", (Tracer, Incendiary, Tracer, SOLID (ball)), there was a LOT of two-way illumination whizzing around in the sky.
Note the "fine print" on the packets that refers to being loaded with "nitro-cellulose powder". MUCH less muzzle-flash than from Cordite. And, as a bit of a bonus, having different burn characteristics, NC powder also burns at a lower temperature, hence less throat / leade erosion per round, per barrel.