Hunter
There are plenty of places. pcspecialist are pretty good, and offer finance if you're on a budget, the configuration process is pretty good as well if your not into teccy stuff:
http://www.pcspecialist.co.uk/
my last pc i bought a mobo/cpu bundle, case, RAM, PSU from Novatech, and nicked the old HDD,DVD from my old PC. it's pretty easy to put them together, took me about 30 mins.
Do not buy a gaming PC from retail, alienware and the like may look very good but they are massively expensive, also people I know that have bought from e-bay sellers have also paid over the odds.
I would suggest that you buy components and build it yourself or get a friend to do it for you, it is easy as long as you are carefull.
Cheers