Why not overclock it yourself?
HERE's an article on how on that board.
I assume you qualify for the free postage on Scan, so to cut costs i'd:
1. Reduce ram to 4Gb
2. Pick a smaller cheaper case as suggested by POB255
3. Buy a slightly heavier duty PSU. Nothing wrong with the one you've picked, I've just not seen any reviews on it and for overclocking you'll never look back once you have a decent PSU. Buy once, (re)use many.
Also, most games are GPU bound not CPU bound, so paying for all that overclock and the rather expensive Corsair H70 over an air cooler doesn't make too much sense if the budget really is the upper constraint. Even if you only manage to get 4.2Ghz out of the processor, you'll see diminishing returns for the outlay.
Other reputable builders:
www.overclockers.co.uk also come highly recommended. Would I buy from them? Only if I didn't know how to. Scan are about as good as they get...