Have you thought about DIY? You obviously don't get tech support, and you won't save a great deal of money ( Dan et al have tiny profit margins ), but you will get the very best components, upgrading is usually easier, and you can get EXACTLY what you want.
I've built a number of systems for myself and friends, and while it is sometimes takes a bit of tweaking to get everything working together perfectly, it is very rewarding.
For £1300 you can certainly build an absolute screamer of a PC, right on the bleeding edge!
If you want any tips, feel free to drop me a line
pilot-lite