Is this a complete build or will you plan on using bits and pieces of an older system.
If its a completely new build its going to be tight for £800, the CPU and mobo will cost 450-500 alone. Im not saying it cant be done, especially if you are willing to concede on a nice graphics card and 6gb ram, but knowing what parts you have already would be a great help.
Your budget is not huge, you don' t game, you don't need multiple video cards, and i7 requires a very expensive gaming motherboard and expensive DDR3. I'd recommend you stick with something designed for non-gamers instead. Try something like this:
Al right, then go for that i7 build, but change the PSu to PC Power & Cooling 750W or to Corsair 850W. I recommended the Corsair 550W thinking you'll have a HD 4670 there. If you're going to replace it with a HD 4870 X2 or GTX 295, or whatever high-end card will come out in summer, then you will need more than 550W.