Hey,
I live in Belfast, Northern Ireland, but was over in New York at Easter. I'm still at school, so buying a Powerbook over here at that time, with the higher price, was out of the question.
So I bought mine in New York, and ended up saving 450 quid all in. I didn't bother declaring it on the way back, and wasn't stopped. Nobody asked anything, and I carried it in my hands (well, inside its Tucano skin, which incidentally, I bought before I went and took it across the Atlantic empty 😉)
Personally, I wouldn't worry about declaring. No doubt some people will feel different, but that's my belief. No one will hassle you for it, so don't worry about that. The PSU is 110-240V variable, as has been mentioned, so get yourself a UK adaptor and you're all set.
Someone else also mentioned the different keyboard. That's hardly a dealbreaker though, and if you're a touch-typer, just set it up for a UK software layout and you're all set.
Incidentally, I just paid cash for mine, on the day. I dunno what I wudda done had there not been any in stock. Just watch out for the sales tax that they don't add on for the American prices mate. They'll add 8% when you're in store, which I forgot about. Didn't put me off though, still made a killing with that price!
Cheers,
andy.
Edit: Oh yeah, do as Vanilla says and dump the packaging! I left mine in the hotel room, you should - and probably will, it's bulky! - do the same. I'm sure you've already thought of that though. You could post the CD's and receipt home, but personally, I didn't bother doing that, and I'm sitting in Belfast with my shiny Powerbook jus' fine 🙂
Vanilla, you said to take into account the cost of a UK powerbrick. To get my Powerbook working here, I bought a 2 quid adaptor at Heathrow on the way home. That was it! Hardly a cost worth factoring in. Go for it mate.