Imports into UK from outside EU are subject to Customs Duty and to UK import VAT. In the past the process for intercepting inbound mail and collecting these taxes was ad-hoc; hence most of the time people did not get charged.
The system has now significantly improved, and most imported goods of any size / value will get held until all taxes are paid. You need to factor those costs (in addition to postage) when thinking of buying something mail-order from abroad.
Take a look at
the HMRC site