They are entirely reliant on misinformation, misunderstanding, naive ignorance, and people 'assuming' things in order to have us pay the license fee. It's all they have because they are no different to any other company in that they can not compel you to purchase their product.
Capita employees have no legal powers above and beyond those of an ordinary citizen in the street. They can not demand entry to your property, they have no powers to compel you to produce a TV license, they can not compel you to answer your front door, and you are under no obligation to entertain them in any way should they appear at your property.
The only way they can gain entry is if they turn up with a warrant, but in order to have a warrant granted they would need to prove to your local magistrate/sheriff that they had grounds to believe you were watching TV without a license. Simply turning up in court and stating a belief that you are would get them nowhere, so they require significant corroborating evidence, and on the rare occasions where warrants are issued, it's invariably because the home occupier themselves has provided some sort of evidence.
Do not engage with them, do not reply to their correspondence, do not let them into your home, and there isn't a damned thing they can do. Their letters are intended to sail as closely as they can to the boundary of appearing intimidating without constituting harassment, but if you know what to look for it soon becomes apparent that they are nothing more than blanket fishing attempts with no consequences for those who see through Capita's shithouse tactics and ignore them.
It's also a complete and utter myth that prisons are full of women who are 'in prison for non-payment of TV licence', but understandably there is no will to have this put to bed either as it just serves to further that fear of being 'caught' and causes people to pay for a license they quite possibly do not need. Non-payment of TV license does not carry a prison sentence. You can not be jailed for watching TV without a license. At worst, you will be handed a fine. In some parts of the UK this is frequently as little as £40-£70, despite the dire warnings of 'up to £1000'. Even then, it is literally a handful of fines handed out in any given year, so evidence suggests that Capita themselves don't really make any serious effort to catch 'lawbreakers'.
What is a criminal offence is non-payment of the fine. This is how the myth arises. Women who are going to prison for other offences in any case, often have an outstanding charge for non-payment of fine. Their legal representative will ask for this to be taken into consideration, so the magistrate/sheriff hands down a token sentence for it to be served concurrently. The women are going to prison in nay case, so this wipes the non-payment fine from their record and the punishment is served alongside their 'legitimate' sentence, so arises this myth that the UK 'jails women for non-payment of TV license'. It's a total misrepresentation of the truth.