You might want to wait a week or so until the new core i5 and i7's come out, or whenever they become available for your country. If anything at all, it should lower the price on some of the older components.
Ideally, an i5 system build would be a better choice since the LGA 775 socket that the Core 2's use is going obsolete. With your 400 GBP budget, I don't think you could get an i5 system at release.
Have you considered building an AMD system? you could still stay within your budget, and not commit your friend to a dead-end motherboard in terms of upgradability.