Simple answer
A simple answer is what you pay in dollars you would pay in pounds when it arrives if your parcel has vat, duty and delivery charges added.
So If the item is $200 and it arrives in the UK by a carrier (DHL or similiar) the final bill including shipping from the USA, UK vat and duty could be about £200.
If it arrives as a gift then you could save the duty and vat - BUT - if it is found that you have miss represented the item then it could be impounded, you could be liable for charges and at worst prosecuted.
Steve