You never clarified that you know the difference between UK and Russia, now could you clarify that you understand B-17 and B-24s were intercepted by GERMAN not allied fighters? The Battle of Britain took place at up to and slightly over 30,000ft. The normal altitude for UK offensive operations with bombers was 15,000 ft. The coast of France and Belgium was a picket line of flak, what are you going to use you P-39s for?Only difference between air combat in western Europe and eastern Europe was there were no high altitude bombers in Eastern Europe. And those weren't really a force in the west until mid '43. Soviets standard combat formation was the "Kuban Stairs" or "Flying Bookshelves" with a flight (4 planes) at 5000meters (16500ft), a flight at 6000meters (20000ft) and the top flight at 7000meters (23000ft). P-39s could match the LW fighters up to 8000meters (26400ft) with neither side willing to go much higher than that.
Turbocharged B-17s and B-24s flew at 25000ft with their escorts a little higher. Not much difference.