One of the immediate problems I have with the British having a 'semi official' air group fighting for the Republicans is that Britain was still clinging to the notion of the League of Nations acting as a force to prevent or subdue such wars a league that which they were a major partner - so while yes such a thing would have helped better develop tactics etc going into 1939+ and I would have loved to see it happen (Extra magic fairy dust points for getting George Orwell to become an ace pilot in said unit) - I just cannot see the British government going for it at the time.I've always thought there was a good POD in there for a better UK performance 1939-41. What if some Sir Humphrey in the War Ministry had the notion of attempting to use the International Brigade (British Battalion) as a laboratory for weapons and tactics. Full deniability etc as a given, but get a cadre of troops from all arms to volunteer (having acquired/stolen) British kit. Likewise I think there were some pilots volunteered, so if the (unhappy recently dishonourably discharged) RAF provided a squadron's worth, even if they were using the Russian/French stuff it would allowed at least someone in 1939 to have (hopefully) learned some of the lessons of WW1 that had been forgotten again: finger four formation, beware the hun in the sun, bombers with escorts etc.
Obviously not all the lessons of Spain are applicable, but as @Martin the Martian said, the Germans did learn some.