, the VAT rate differs between countries. However, for the UK assuming you plan on permanently importing the boat then the Queen will take the going rate of import
duty, including the cost of shipping
, plus VAT at 20% on top of all that. This will give you a flavour of what is required:
HM Revenue & Customs: Customs Duty, Excise Duty and Import VAT: introduction
You would also need to have the boat brought up to CE spec.
this will help explain the problems with importing a US boat:
CEProof | american imports (ASB)
In general then, when you add up the cost of permanently importing and then changing hardware
from say 120v to 240v it is going to expensive. As Protectingtheocean says, a temp import and then exporting the boat is a lot easier.