If you don’t pay for it, you get a harassing letter every month. You can find them documented here.. Occasionally, someone from TV licencing may visit. You have no obligation to let them in.

Most people who do get convicted are the ones who engage with TV licensing. For example, they admit to using the iPlayer. They miss a payment on a weekly payment plan for it and then say something like “My mum was in hospital and I wanted to keep the kids entertained”. Someone I know almost got caught out because they tried to do the “right thing” and sent them an email saying they didn’t need a tv licence (they were doing it on behalf of their elderly father. They personally owned a licence) and TV Licensing detected their email was attached to the BBC iPlayer. Or they were let into the house and set up the TV for them.

If you just ignore the letters, and shut the door as soon as they show up, basically nothing can happen. There is very little way they can prove you were watching TV. Someone else mentioned “detector vans” but these were most likely fake (The BBC claimed they did that but didn’t let anyone see how they work, even the Ministry of Defence who was very interested in such a technology), and definitely wasn’t admissible in court.