I've ordered stuff from the US in the past and it seems hit and miss whether you get caught by customs for small items but for valuable items, you almost certainly will. You can expect to pay VAT plus import duties (I'm not sure what the latter are, you will need to check with HM Revenue & Customs). If the sender uses the postal service, the Royal Mail will hang on to your item until you have paid duty plus a "handling fee". The latter seems to be somewhat excessive, at least on smaller items as it can be as much as the duty and it's that that I particularly resent. I have heard that courier firms tend to be more reasonable in that regard, so ask the sender to use a courier and not the postal service.
So the short answer is, yes, you will get charged by customs but you will need to check exactly how much.
If it's sent from within the EU, on the other hand, as long as VAT has been paid locally by the sender, there should be no further duty to pay.
