U.K. residents only. In the U.S. the best I've seen is 1% cash back either with Fatwallet or eBates.
Yes but the macbook is like £200 more expensive here!
Yes but that is mostly taxes. The U.K. pre-vat price is approx $70 more than the pre sales tax price in the States. The 6% cashback would more than pay for the $70 so U.K. residents can actually gets it cheaper than the U.S. on a pre-tax basis. (If you don't like taxes contact your M.P., not Apple).