I’ve been out of the loop for a couple of years, so I’m not sure of the different model numbers et cetera. However, it’s usually much cheaper to buy the parts individually and build the machine yourself (the opposite is true with bikes). I can recommend Scan for parts.
As above, a top level I5 will serve you well. RAM costs next to nothing, so knock yourself out (although you’ll need to check whether your OS can utilise more than 4gb). You can save quite a bit of money by using a decent graphics card from the pervious generation, which will give you an upgrade path should you want it. Also, I’d strongly recommend installing your OS onto an SSD.
All in all, you should be able to build a top quality PC for around £600 or much less if you’re willing to make intelligent sacrifices.