I'd definitely buy it in the US at this time. (I speak from frequent personal experience in Austria, but I suspect the UK situation is not too different. Check with customs, however, to make sure.) While smuggling it into Europe (yourself or via a friend) is not that hard, its a risk I never wanted to take, both for its own sake, and also because I write my machines off as a business expense in Europe, which you cant do with smuggled items. Legally importing it from the US, including shipping, insurance and duties (youll probably have customs on the total cost, including shipping and insurance, and VAT figured on that amount) is usually still cheaper than buying in Europe; all the more when you can write it off; and now, with the Euro/Sterling bonus, it seems to me a no-brainer (thisll probably get even better over the next few months, so theres no hurry).
If you buy from the US Apple store, youll need a US address (a friend), from which it will be shipped on to you. Easier on friendships, more efficient, and frequently cheaper: many of the US mail-order houses will ship to you internationally (and accept your UK credit card), and if you can get the hardware package you want from them, its probably the best bet (checking around, you may be able to find some which will outfit it to your specs). You can also get any additional stuff (hard- and software, books) from them, too, in the same shipment. Hardware and software is probably taxed differently, so ask beforehand.
Caveats:
1. Voltage and plug style: check the original specs before you buy. Nowadays, almost none of the hardware runs on pure house current, but the question is whether the US power supply will accept up to 220 V. It probably will, so all youll need is a standard plug converter, widely available cheaply. However, if you have to actually buy a new power supply, it might be cheaper buying the whole item in the UK (this wont be the case for the computer, but may be so for, say, an AirPort base station).
2. AirPort: get you AirPort and AirPort card in the same country. While this may have changed, a few years back there was some question whether "hybrid" setups would work, due to allowable frequencies here and abroad. Actually, they may, but I wasnt willing to risk it.
3. Remember, youll end up with a US keyboard and US software. The problem in the UK, if it arises at all, would only be with the ? (pound) and the (euro) symbols. Of course, you can type these even if theyre not on your keyboard, but it takes some getting used to, and is a bit irksome. This may no longer be a problem today, but was in years past.
Good luck with your new machine!