Thanks for the help guys.
Dan, I have looked for one with XP as that is what I am running now, but I haven't managed to find any.
You won't find any new PCs with XP as Microsoft won't allow companies to sell it. Businesses with certain licence agreements can get one with Win 8 and then remove Win 8 and put XP on. A lot of businesses are looking to move off XP as the extended support available to companies ends on April 8th next year. This also means that security updates and patches should stop.
I wouldn't recommend XP and SSDs as XP doesn't fully support all the capabilities of an SSD, something called Trim, although there are software packages to get round it. You may also find the XP installation a problem unless you have a little knowledge of drivers and how to add them at setup, if your new hardware still supports XP and you can get drivers.
If I were you I'd stick to Win 7 or 8. There is something to be said for buying a complete box as margins are so tight it often only works out cheaper if you buy parts in bulk. I build my own with quality components so I know what goes in them, but I often think this is more expensive.
A Windows 7 oem licence from somebody like Scan is £70. That only leaves £200 for memory, processor, case, psu, hard drive, Blu ray and system board with on board graphics. It doesn't take into account other things like better graphics and a possible tv adapter.