Customs will charge an fee for imported goods, the fee is dependant upon the catagory of goods imported, everything from Elephants semen to Cowboy hats has a duty rate, these are listed in the import dury rates tariff, the calculations for the amount payable are reached by taking the value of the goods plus freight, multiplied by the duty rate, then multiply that amount by 20% to add the vat, but the costs dont end there, a customs clearance fee is usually charged.
As has been pointed out these charges can be avoided if the sender declares a low value and that the item is a gift, but that means the sender will be making a misdeclaration and breaking the laws of both the country of origin and the country of destination.
Bottom line, if you cannot get the item of your desire in the UK, then be prepared to pay these extra charges.
I have over 20 years in the Import / Export business so if you need to know more, pm me