Apart from Jimmy Grimble (which is number 1 for obvious reasons), Britain's best at making comedies, rather than all-out serious films which invariably end up being Downton Abbey type shite.
So Mike Bassett
David Brent: Life on the Road
Four Lions
The Inbetweeners
Lock Stock
Snatch
Dr Strangelove (although arguably more American)
Trainspotting 1+2
Sacha Baron Cohen films
Kevin and Perry Go Large
Of the serious British films and of the ones that probably won't be mentioned, I thought The Ghost Writer was a good film, one for the conspiracy theorists on here.