Hi Chris, I buy parts in from USA for an old '57 Chevy I've got and at $2 to the £ it's a good time to buy from the USA. Credit cards charge a fee for foreign currency transactions as well.
So say it's shipped to UK via UPS (expensive but tracked and you will get it in 3 days!) for about $100 ( guestimate) your now looking at $5000 = £2500.
When it gets here you will be liable for import duty at I would say about 10% ( contact Customs and Excise for correct rate), £250.
So now its at £2750.
Then VAT is added onto the whole amount, £481.25 giving total of £3231.25.
If you're vat registered then you can claim it back.
HTH, Martin.