Hazuki and Skyline have some recommendations in this thread:
http://www.lomcn.co.uk/forum/showthread.php?71764-Computer-Components
Also, I wouldn't even consider building a desktop today without at least 4GB of memory (unless it's for a specific use).
I am not sure about pricing in the UK, but here the i3-2100 is a fantastic CPU in terms of cost/performance. If you don't want to spend that much on a CPU, you could always buy a Intel Pentium G series CPU (socket 1155) and upgrade later on to an i5.
Alternatively, if you just want to build it and get it over with, an AMD Phenom II X3 should be in a lower price range than an i3 (socket 1155).
Be aware though, AMD's new "bulldozer" CPU's are coming out before the end of the year. This may reduce the price of the current Phenom II series processors (but more importantly probably offer a new, bottom end CPU that will probably be faster than anything at it's debut price point). So, unless you are pressed to buy something now, I'd start saving and get yourself something around October or so that is a high performance to cost ratio.