Ok haven't done one of these in a while but best performance per pound atm -
AMD Athlon II X4 645 - £75
HD6770 - £85
Gigabyte GA 88GM USB3 - £70, has latest USB, and most importantly paying the ~£20 extra for an AM3+ compatible board means you can upgrade in the future to an AM3+ CPU as opposed to needing a motherboard upgrade if you want the next generation in the next few years
Samsung SpinPoint F3 1TB - £40
2x2GB Corsair 1600MHz RAM - £25
NZXT Beta Evo - £35
PSU - I'll let you choose but should be between 400-600W for this type of system and MAKE SURE you buy from a good manufacturer, Corsair, XFX, Seasonic, etc. you can look them up. (Corsair CX430W should be sufficient for this build and leaves room for upgrades)
Optical Drive, choose any old one just make sure it has warranty. Samsung Writemaster is cheap and has never failed me
CPU Cooler - if you plan to overclock, get one like the arctic cooling freezer 7 pro rev 2 - £15
Approx cost - £350 w/shipping, if you have more to spend then for the CPU an Intel i3-2300 with a P67 board (this type of board supports overclocking if you decide to upgrade to a "K" edition CPU which are the only Intel CPUs from the LGA1155 architecture that support OC'ing). For single threaded games the dual core will be good and should hold its own against the older AMD quad core even in multithreaded games (e.g Turn based strategy).