TV detector vans exist, but they do not detect anything. They are just for show. TVL have a database of addresses in the UK with or without a licence. It is just assumed that anyone without a TV licence is guilty, and so a campaign of harrassment begins by letters and visits to intimidate people into buying a licence. Fact 1: Never in court has evidence been used to prosecute people (mainly single mothers) based on 'detector' van evidence. Fact 2: Although TVL threaten that they might apply for a search warrant if you do not let them in to inspect your house, it is very difficult indeed for them to apply for a warrant. They first need proof that you are receiving live broadcast. Never has 'dectector van' evidence been used to apply for a warrant. Search warrants are very rarely applied for today.
The reason people get prosecuted is that they admit to having a TV and then sign a 'confession' form. They assume a visiting TVL 'officer' has some sort of legal power when they visit. They have no more legal power than if any member of the public came round. Tell them to leave and they must immediately comply else be in breach of law.
Philip Dean, Newport, Wales
Source: My mother was convicted many years ago, I was there and witnessed the entire confession-based playbook.