The law in the UK states that a licence fee must be paid yearly by every household with a television. This is collected by the TV Licensing Authority (TVLA), a private proxy of the BBC, who use the money to fund their services. While the rights and wrongs of this are debatable, I will concentrate on how this affects the minority who don't watch TV.

From "About TV licensing" chapter 6. Item five.

5. What if I do not have a television?
We have a statutory responsibility to ensure that every
address in the UK where a TV Licence is required is
correctly licensed.

The government allows them to collect fees from all TV users. "Responsibility" and "duty" are words used to fend off every complaint about TV licensing policy. See, for example, the parlimentary committee debate linked below.

We do this by writing to people to
ask about their use or otherwise of television.
If people reply to our letters to the effect that no
television is used at their address, we place a stop
on further enquiry letters...

Some years ago the infamous TV detector vans were used to try to catch licence fee evaders. They have obviously figured out  that sending letters is cheaper. The letters are an odd mix of PR, information and intimidation. They obviously have never considered that sending unsolicited demands for information is, at best, very rude. Still, most people could live with ticking a box and returning a reply slip.

...and arrange for a Visiting
Officer to call upon them to verify the situation.
Visiting Officers should act professionally and
courteously at all times and their visit should take
only a matter of minutes.
They will seek permission to enter a person’s property
simply to take a brief view of the main living areas.

This is where things turn slightly Orwellian. The right to privacy and freedom is cherished in democratic countries, and even the police require judicial permission to search someone's home. Yet this private company expects that most people will waive their rights under pressure.

If the Officer verifies the fact that no television is used,
we will then cease all forms of enquiry to the named
person at their address for a period of 3 years.

It gets worse. They expect to search your home not once, but on a periodic basis.

The occupier is under no legal obligation to allow
entry but it does enable us to place the long-term
stop on further enquiries.

Of course it's not legal. The company expects to compensate for lack of legal powers with harassment and intimidation of the meek and ignorant.

The reason we have adopted this policy is that in 2003
alone one in three people who claimed not to have a
television were found to be using television when TV
Licensing checked the premises, which makes it
necessary to visit in order to verify their statements.

Ha. Imagine making this argument to a magistrate. "Your Honour, surveys show that one-third of the people on this housing estate take drugs, we'd like a search warrant for every house."

We appreciate that not everyone who claims ‘No TV’ is
a licence fee evader, but we feel that the fairest and
most consistent approach to the situation is to call on
everyone who makes a ‘No TV’ declaration.

This is the worst yet. They may appreciate that some people don't want TV, but they have no appreciation or respect for those people's rights.

Another favourite trick not mentioned above is to read the householder a legal caution to attempt to gather evidence for a court summons.

You do not have to say anything.
But it may harm your defence if you do not mention when questioned
something which you later rely on in court.
Anything you do say may be given in evidence.
Do you understand?

Now the householder is being interrogated on his own doorstep by a corporate agent just because he doesn't have a TV. Only in Britain...

I'm not sure what they do when someone ignores their blathering letters and/or refuses to speak to their officers. They say they "look for external evidence" such as a TV aerial (because when someone moves house they always take the TV aerial with them) and then "think about using detection". Interestingly, the TVLA keeps secret the number of detector vans they have, probably because the number is embarrasingly small.

You can find on the internet horror stories about the TVLA being granted search warrants on the flimsiest of evidence, then breaking down people's doors. It's probably best to take this with a pinch of salt, but I wouldn't entirely put it past them.

Further reading: