Hi mate im from uk too, the best uk sites are Ebuyer, scan, there the main ones, if something isnt on Ebuyer you'll find it on scan or vice versa. Ive never had experience of scan but i know of people that have and i think there a good site to order from, ive had 3 items off Ebuyer all on next day deilvery with citylink and i got the items on time and without dents or scuffs etc, so id recommend them.
If you have the cash id recommend this one Hiper 580W Type-R Modular PSU SLI/CrossFire Certified - Black
Plenty of power for upgrades later, and its also modular which means you wont be putting a spaghetti junction of wiring into your case! you will get several cables in a seperate box, you just then use the cables that YOU need, the rest stay in the box, hence a better looking case with less cluttered cables and better airflow.
Any more questions feel free to ask