I was in the same boat, with the same budget a few weeks ago, and couldn't find much, but came across a Carrera Vulcan in Halfords, for £320, I usually stay clear of Halfords but the reviews were too good to not at least have a look. For the money it is pretty dam good,if your doing a lot of street riding I'd change the tyres though as the standard have such a high rolling resistance.
The brakes can be a pain to bleed to, as I've just found out, but mine are finally fully working and set up correctly, and are very good. I posted earlier that they're crap, that was before I sorted them out, I'm surprised on how good they are for an entry hydraulic disk brake.