The United Kingdom has the strongest air force at 1939. The Royal Air Force, complete with fast and deadly Spitfires can wrestle control of the skies from any of the other nations. After a fifty dollar research fund for Spitfires, they are cheaper than ME109s and the same price as Zeros and did I mention Spitfires will win a one on one with any other aircraft at 1939? The Wellington is one of only two non-light bombers at 1939 (the other being the very powerful but very slow and vulnerable Russian PE-8) and can destroy any production building in a single run. The Hurricane, while not as powerful or cheap as the Stuka, is cheaper than the only other 1939 fighter bomber, the Italian Sparviero, and one equal footing with everything other than the ME109, Zero, and Spitfire in air to air combat. The UK also does not need to research their paratroopers which are better than their land infantry in every way.
The bad news? The UK has the worst ground units at 1939. Your Regulars are inferior to everything but Russian Strelki and are ineffective against armor thanks to their PIAT mortar launchers. Your Cruiser tanks are basically more expensive Stuarts and your Matildas are well armored but poorly armed. Finally, your Artillery and anti-air are very expensive for their abilities.Focus on providing air support for a more powerful land nation like Russia or France instead of your own ground forces.