Owain Yeoman (Rigsby in The Mentalist) is a Brit, he's Welsh.

Lennie James (Richard Hawkins in Jericho and Charlie the pimp in Hung) is also a Brit, English this time.

Not a Brit (well, maybe an eighth or ninth generation Brit!! ;)  - just kiddding! ) but mentioned as someone else mentioned NipTuck earlier is Julian McMahon ( Dr Christian Troy in NipTuck, also Cole Turner in Charmed and Dr Victor von Doom in the Fantastic Four movies).