Intel Core i5-2400 (not overclockable, but only 10% slower at stock speed than the ever popular -5-2500k)
An ASUS or Gigabyte z68 motherboard. Just pick the model that best fits the budget and the features you want/need. All are good from low to high end.
8gb(2x4gb) DDR3 1333 ram from Corsair, Crucial, Kingston, or Gskill. You do not need the fancy heatsink type either. The plain "unclad" style will work just as well. You will need 64 bit Windows for this amount as well.
The hard drive will be whatever you can get due to the shortage, but a Western Digital 500gb SATA III 6gb/s would be a good choice.
Video card - The Nvidia 560 Ti or AMD 6950 is an all around good choice these days. They are basically equivalent. Here is a handy tool to see how the 560 Ti will perform in various games. You can extrapolate over to the AMD card.
http://www.geforce.com/Optimize
Power supply - Since you are fortunate to live in the UK, you have access to a very good PSU deal. I have been recommending these for over a year now and never a complaint. Wish they sold them in the US.
http://www.novatech.co.uk/products/components/powersupplies/nov-psg600.html
Case - These are more of a personal preference, but any good mid to full tower with good airflow and at least one 120mm intake fan, and one 120mm rear or top exhaust fan is preferable. One popular type right now is the Coolermaster Haf series. Some only come with one fan, but extras are only 10 pounds or so.
A DVD RW can be had just about anywhere. I am partial to Samsung.
I would check Amazon UK and Novatech as the best price options.