Everybody knows about Massive Attack. They basically led the way and pioneered the course for future triphop artists. Their latest album "Mezzanine" is a little dark but is still very creative.
Listen to Massive Attack
Visit massive Attack's label Melankolic. It features Alpha, Parker Lewis, Horace Andy, and Craig Armstrong.

Portishead is out of Bristol, England, and followed Massive Attack in the triphop emergence. Their two albums "Dummy" and "Portishead" consist of ethereal vocals over smooth bass and drum lines.
Listen toCowboys
Listen toWandering Star
Listen toStrangers