I wouldn't buy anything under 500W - there's just no point. If you decide to upgrade a graphics card in the next few years, you don't want to have to buy a new psu as well. The technology within psu's hasn't really changed much in the last few years so anything you get now is pretty much future proof. Stay clear of the crapmasters (coolermaster); corsair is a very good brand: http://www.amazon.co.uk/Corsair-CMPSU-600CXV2UK-Builder...
Also when choosing a psu you should look for one which is 80+ certified (less of the total Wattage is lost as heat).
Have you considered getting components off amazon.co.uk? Stuff is generally cheaper and also you get a better selection of cases. On www.scan.co.uk you can get the CM 690 lite for under £60 and it is a seriously good budget mid tower case.
The corsair carbide 300R is also under £60 and is very good in terms of quality, space and cable management features.
+1 to the XFX not very fond of OCZ so it's more of a personal preference not saying it is a worse psu i just trust XFX more and since every XFX PSU it's actually built by Seasonic all the more reason i would get it am i a seasonic fanboy maybe but that's besides the point lol