It's legal, if you declare it on import, but then you get hit with a 17.5 % (I think) tax, which, with shipping usually pushes it to the realm of not really worth it. Of course the more expensive the goods, the more a saving you can still make.
Of course, you can simply buy one in the states, or have someone visiting the states buy one for you, and not declare it, but you can still get caught and taxed, and there is the warranty issue. I think I' right in saying the US warranty isn;t honoured in the UK.
My main problem is that if something is found to be wrong with the camera it's such a pain getting it fixed. If you buy it here, at least you can simply take it badk to the shop.
OF course an ideal solution would be to buy it while on holiday, in the EU someplace (UK has EU warranty). This would give you plenty of time to check the camera out for faults, and return it if neccessary. IT's just finding somewehre where it's good and cheap in the EU. I heard Angorra is good...