I own a Graham Slee "Solo" amp, and it's very good. Buy directly from Mr Slee at
www.gspaudio.co.uk .
Stone Audio (
http://www.stoneaudio.co.uk/ ) will sell you a Sugden Headmaster, if you want to go a bit more upmarket. (That device also doubles up as a quite nice stereo pre-amp). Stone Audio also sells the Earmax, which is a very fine headphone amp that runs on valves rather than solid-state circuitry.
Those three (Slee Solo, Headmaster and Earmax) are the "usual suspects" for people looking for a high-end amp.
Superfi (
http://www.superfi.co.uk/ ) sell Creek amps, which are cheaper and reasonable value.
Some people like the X-Cans amp, although I'm not a fan myself.
There are various other possibilities - do some
Googling and
Froogling.