HE discount was worth about £200 on my base 27″. Buying that way gets you a 3 year hardware warranty (not full AppleCare inc telephone support, etc). As said, the back-to-school offer is on so you get £60 of apple credit for apps, music, etc. You just have to be a student so befriend one or take a short course at the local college.
Buying in the US attracts local sales tax (about 7% in NYC, iirc). Not sure on power supply but I suspect desktops may not auto-switch like the laptops do. And in terms of practicalities, it would have to be checked into the hold and your travel insurance is unlikely to cover a £1200+ computer getting broken in the hold. Shipping it with insurance will be pricey and guaranteed to attract VAT when it arrives. And little chance of customs missing a huge Apple branded box.
Best option aside from the edu discount route is Dixons at the airport. Can pick up on your way home or get it delivered. Can check what the current discount is on iMacs when I fly on Monday if you like?