Never imported a guitar, but I've bought a fair amount of stuff from the US before. Like Dave_Mc_2 says, guitars come in at about 22-23% including VAT & Import Duty. I've looked seriously at importing stuff to the UK in the past, but always managed to find the stuff I was after in the UK, well most of the time.
If you do import an amp from the US, you'd have to remember to either buy one with a power switching Mains Transformer, the US uses a 110v power supply, where the UK uses 240v mains. If the amp's mains transformer is US spec only, you'll have to get a Step Down Transformer to convert the UK's 240v to the US 110v. You should be able to get one from B&Q or any other DIY/Builders Merchant (UK building sites use 110v).