You'd have to check the voltage rating on the transformer/Wii. I'm sure USA are on 115v as opposed to the UK's 240v. Plus, the console will only play USA region (1 I think) games and likely output in NTSC, so you'd need a TV capable of taking an NTSC input (Europe is PAL) and you'd also need to make sure the games are the correct region (ie, USA).
In short, for the few pounds you'd save, it would just be easier to buy one from the UK and have the peace of mind of having a warranty.
/2 pence
if you're a good enough friend of mine, I can get one in december - they're a good 40-50% cheaper in Saudi. I can get a PS3 for £340 (as opposed to £550 here) in Saudi