For the Brit tech tree, rush to get the Spitfire 2b. It's by far the best of the level 2 British planes, and is so good that I still am using it in level 5 matches. Some of the advanced killers on here I believe have small shrines to it setup in their homes
If you have Golden eagles to spend, get yourself BOTH the Boomerangs and the D520. They are also great planes to battle in.
Learn how your firing distance works (Short distance if you want to dogfight, long distance if you want to boom'n'zoom or bomber hunt), and consider locking your cannons and machineguns to seperate fire buttons.
Fire ahead of the little ball marker when playing arcade. The further away you are, the further ahead of the marker you need to aim. Enemy speed and your firing angle also come into play, but that's an art, not a science. Keep trying and you'll get it. But you can also fire at one spot ahead of the marker and let the marker pass through it. This will almost guarantee a hit, but you need it to be a good one to score a kill.
I consider the British bombers, after the A20G, to be crap, so I would focus on your fighters if you can.
Use Combat flaps (F button) when dogfighting. Your turning circle will reduce and increase how deadly you are. But your speed will drop faster too, so don't overuse it.
and finally... Come join our squads!