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.
Go with a G41 board as it has better onboard graphics (still crappy but a big step up from the G31) and official support for DDR2 1066 without having to overclock the onboard memory controller. If you were going to use a dedicated GPU then I guess the G31 would be fine, but if you want to use the onboard GPU it won't be too fun trying to use Areo, but it would be just fine on the G41.