The first Brit in space was Helen Sharman on Soyuz TM12 in May 1991. The second was Michael Foale.
Helen Sharman is the only one to stay a Brit though, Foale and Sellers both had to become Americans and join NASA. I can't think of any that have tried becoming ESA astronauts, plenty of French and Germans have flown this way on the Shuttle and Soyuz.