You will generally be hit by:
Import Duty: variable % depending on what HMRC choose to class it as
VAT: at current 20% rate
Handling Fee: usually around £10 depending on the carrier.
Often the carrier will pay these fees and then bill you before releasing the item for delivery.
Other times it will be held at customs, or by the Royal Mail if they are the carrier.
There is also the off chance that HMRC will class it as a firearm, destroy it an then send you a letter telling you what they've done....
Add to that if its a new marker, you'll have to have its warranty fulfilled in the USA...
Always, always buy UK - it supports our market and saves you hassle. And you'll be surprised how little you've probably saved yourself in the long run....