Click the select button under the 27" i5, and it will then bring up a page where you can change/upgrade parts etc. The i7 should show up as an option under the processor section.
The i7 is only built-to-order so click "Select" under the i5 iMac and it lets you customize it with i7 processor. The only difference is the CPU though. i7 costs 160£ more than i5, meaning a total of 1794£
I came to the conclusion that, for me, I do come video encoding, and gaming. The graphics card is the bottleneck for what I do - soI went with the i5 ass it's quite a performance boost over the C2D