I've bought a few things from the US, and I can say the following:

Import Duty is payable at 30% on most things, but may be included in the carriage costs. For CDs they don't always notice/bother. When they do notice, the carrier pays for it and then collects from you before you get the goods. If the Royal Snail is involved the system charges it as postage due. The postman tries to collect it if you're in, and if you're out leaves a card to tell you to go to the sorting office and collect the parcel. If you're buying something large/expensive, find out how to insure it in transit from the UK (buy a freight agent a pint or two), then instruct the US company to underinsure it to about a fifth of its value. This will reduce the duty, which will be applied to the US insurance value. Also, make sure it enters the UK anywhere but Heathrow, where they have expert assessors to combat this dodge. Manchester, Bradford, Luton, Newcastle and Glasgow are good places to import vintage guitars.