I thought customs just check some shipments and if they see something illegal coming through such as high powered lasers or explosives they confiscate it and send you a letter. But if they can't test it, they can't legally confiscate it. I'm not sure how strict customs is in the UK.
The principle job of customs, in the UK Her Majesty's Revenue & Customs, formerly the inland revenue, is to raise revenue.
Goods bought outside the EU can be bought without sales tax (VAT,) ie duty free, if they are to be exported, but are subject to duty on arrival in the UK.
All countries operate a similar system, the EU is a bit different in that it is treated as one county for sales tax purposes.
Transit countries are not affected, only the final destination.