If you claim it in customs, then yes, you're likely to get hammered (although the difference in cost may still end up being less than the duty you pay). I've been through Heathrow eight times now and never been stopped in customs (then again, I've always come through as a tourist and never had anything to claim). I don't know if you can file a form to get a refund of the sales tax you paid in the states as I've done in the EU when leaving, but if you've got to pay ~8% in sales tax and then 15% VAT as duty, you'll probably just about break even. Just a guess.
Otherwise, yes, your MB will work fine in the UK. You can even get a set of international plugs from Apple that will adapt the power brick to look just as if you bought it back home. Keep in mind however that the US keyboard has a slightly different layout. You can certainly configure OSX to think you've got a UK keyboard, so for example, you won't have to hit option-# to get the £ key and shift-option-2 to get the € as I do, but it will look different than you're used to.