There is import duty and then VAT is added to the purchase price + duty. Rate of duty is set by Customs (different rates depending on category they choose).
Attempting to smuggle something in without paying duties is of course a criminal offence (and always remember Customs do not need a search warrant and can make a search even after you have arrived home).
If you import something and pay the fees remember to have a copy of the forms if you ever leave and re-enter the UK again as without them a Customs Officer can charge you again.
Pretty sure the offence will also apply to someone who buys such a scope in a private sale knowing it to be imported without declaration.