IIRC, you only have to pay for a TV license if you watch BBC stuff as it's being broadcast.
You have the option of saying you don't need a license, in which case they send someone around to make sure you aren't lying. This is how it is currently at my place (althought they never got around to sending their van over..)
The "detector vans" are FUD, what they actually do is just assume that everyone has a TV and send them a letter. The TV licencing company (and it is a company, not a government organisation) is quite misleading in the way it practices, their inspectors cannot for example enter your property unless you let them in. And they can't force you to answer any questions. So there's no way for them to prove you're watching TV without a licence. All that said, personally I think the TV licence is awesome value for money, BBC TV and Radio programmes are generally of very high quality, and of course there are no adverts and there are very strict guidelines on impartiality etc
You have the option of saying you don't need a license, in which case they send someone around to make sure you aren't lying. This is how it is currently at my place (althought they never got around to sending their van over..)