Look on ebuyer and scan.co.uk, you'll save a load of cash.
Also don't get the Asus M4 series AM2+ mobo, it's just an everyday AM2+ mobo that's overpriced, if your willing to take a slight risk then the MSI K9A2 Platinum is £100 inc vat but otherwise it's got the same specs. Or if you don't need Quad CFX then get one of the Gigabyte CFX boards.
I know, I just want make my rig more future-proof, so i don't need buy anything so i really need a help on rig.
£800 and a future proof PC is possible. (future proof = not needing future upgrades)
£800 and a future proof core i7 PC is not really possible. You have to cut too many corners and you end up with what is a "crippled core i7" system.
I can do some searchin around for places to buy from in the uk for some products on i7, if your wanting it but, a low end i7 wont be worth the build against a PII budget build or a high end Quad build might compare. Then again, you could buy the low end i7, work with it for a while and upgrade it part by part but, i think that would just waist money.