If you are looking at a budget of £600, then I would seriously look at getting a secondhand Hotplate (about £175), leaving you £425 for an amp. The Hotplate would really open up the possibilities of good lower volume tone from a tube amp of around 30W. 30W is definately plenty to gig with (if you're being miked through a PA), but also tameable with a Hotplate for lower volume at home.
Something like a Hotplate with a Classic 30, VC30, or even a Spider Valve 1x12 (which I have heard plenty of good things about if you are able to push the power amp at all - whci you would using the Hotplate).
One thing I'm not sure of is whether any or all of these have easy access to the signal path between the power amp and speakers, for inserting a Hotplate.
Alan