Early Allied campaign
Don't just jump in. Plan, Recon, Plan! Then repeat. Figure out your strategy e.g. oil, fuel, ac production.
Perform recon that fits your strategy. Also airfields, often. Remember the AI can move units as often as you can, and it will.
Prepare your units. Move them to airfields where they are effective. Move short range fighters to the coasts. Personally I place 2 Typhoons and 2 Escorts(spitfires) at airfields along south coast of England.
Ok you have done moves and Recon...time to bomb! Wait. Remember initial Axis response will be heavy. Allies found this on the Schweinfurt raids. I suggest some heavily escorted short range raids using medium bombers. Wait about an hour launch short range raids w heavies and some fighter sweeps against airfields. Wait another hour and launch your longest range raid. Make sure it is heavily escorted. At least 1 Fg every 10 minutes to target and every 20-30 min on return trip.
You will take losses, lots, but you will create havoc among defenders.
Scale your raids to target, say 4BG to small targets, 6BG to medium targets and 8 to longer range, larger targets.. watch morale, rest units as necessary.
If you sustain this for a period of time you will see gradual degradation of quality of defenders. Good luck