Although most detection is done by database the Beeb still claims to maintain a fleet of vans which can tell if a particular address contains a TV. If you buy or rent a TV or buy a PC tuner card in the UK your address is passed onto TV Licensing. The BBC maintains a database of 29 million residential and business addresses which it can check for up-to-date licences. If there is no response to letters an "enquiry officer" is sent to the address. If householders refuse them access - which is quite legal - then a detector van will be sent.
Just a reminder that the British subjects pay a license fee for BBC, which means it is a public trust. For BBC to carry out Israel's wishes regarding the Gaza fundraiser is a theft of service from the British people.
Send the lot of them to The Tower! (I'll pay the license fee just to watch that!)