Formula SAE started in the USA back in 1981 so the British universities have a lot of catching up to do. Added to that many of the overseas universities seem to place much more emphasis on the event than perhaps UK universities do. While that is starting to change as the event becomes more well-known, it's hard to compete against the larger and better-supported teams from America, Australia and increasingly Europe.
That said, Oxford Brookes came 3rd overall last year and the top UK entry this year came 6th (from the University of Bath, I'm proud to say) so the UK teams are certainly competitive.