Man, this would all be so much simpler if there was a good UK equivalent of Newegg. I've searched for ages but haven't found a good one.
I'd recommend checking out scan.co.uk, ebuyer.com and overclockers.co.uk. I built my system fairly recently and I picked up all the parts from those three sites. Scan's generally the cheapest but ebuyer and overclockers do some pretty good special offers every now and again. I don't really have much more advice for you that hasn't been mentioned already, I'd look into picking up a 600-750 watt power supply. I've always used Corsair PSUs and I've never had any issues with any of them. As far as cases go, just go for something that you like the look of but check that your motherboard and graphics card will fit first.