Each different type of airplane had something to contribute to "lessons learned" ..... even if the lesson is "turrets don't work on single-engined airplanes."
Build small batches of most OTL airplanes and test fly them until weakness are discovered.
Simplify Training Command by standardizing on a few monoplane trainers. Follow the RCAF's lead in standardizing on Fairchild PT-19 .... or one of the half-dozen similar British designs. Make them out of plywood to see pressure on traditional aircraft manufacturers.
Encourage Rolls Royce to continue perfecting Merlin and Griffon engines as early as possible. To help that effort, send Hooker to RR as early as possible.
Scrap Vulture and Napier Sabre engines early on. Fund Whittle properly during the 1930s. Staff his machine shops and text cells with the brightest technicians and give him enough engineers, accountants, etc. to handle mundane chores. And for God's sake loan him an obsolete bomber to test fly jet engine prototypes.
Encourage Bristol to build Hercules engines by the thousands.
Build small numbers of Bolingbrokes and Beauforts and Beaufighters early on, but shift those roles to Mosquitos as soon as Mosquito production is sufficient, replace all the Beaufighters with specialized Mosquitos.
As Bristol phases out light bomber production, get them to work on upgrading Bombay transports. Encourage them with rough sketches of Bombays, Skyvans, Aerovans, Hercules, etc. until Bristol perfects a transport with a nose-wheel and rear cargo ramp. With sufficient incentive, they should be able to develop Low Altitude Parachute Extraction Systems in time for the 1944 invasions.
Hang cargo panniers under old bombers: Whitely, Stirling, Halifax, etc.
Kill Albemarle early on.
Train more RAF pilots as forward air controllers and forward artillery observers and send them on the ground with the leading platoon in every infantry regiment. Equip them with armoured cars and a variety of reliable radios.
FACs attached to the Parachute Regiment should accompany them into battle, arriving by parachute or glider.
Speaking of gliders ..... start earlier and more vigorously developing cargo gliders with rear ramps. Tow them with obsolete bombers.
Definitely concentrate on ASW. First, encourage Avro to develop Shackleton earlier. Secondly, develop a monoplane with 3 crew and an enclosed cockpit that can fly from the same escort carriers as Swordfish. Navalized Lysander???? Radar and radios are more important than torpedoes. Maybe a 20mm cannon or two to discourage deck gunners.
Speaking of 20mm cannons ... install 4 of them in every fighter as early as possible.
Quit wasting time installing .303 MGs in bomber turrets. A pair of .50 cal. Brownings should be the minimum for turrets on bombers. And install fewer turrets. When you remove the smaller guns, improve windows, heating and other crew comforts.
Install tail-warning radar, but try to develop frequency-hopping radar as early as possible.
Retire Bomber Harris once he has Bomber Command up to speed. Try to minimize terror-bombing of German residential neighborhoods. Instead, train bomber crews to aim for bridges, railroads and canals because if Jerry cannot deliver weapons to the battlefield ......
As for fighters, phase out Hurricane production. Encourage Supermarine to develop the Sea Fang much earlier.