From the sounds of it, this is just one of the standard letters they send out to people. If you don't then buy a licence, they'll send increasingly threatening letters which vulnerable people could interpret as they are being taken to court and be scared into buying a licence they don't need. It's scandalous but they've done this for decades.
I've notified them before that I didn't need a licence (no TV for many years, then didn't watch live TV). The letters would stop for a while then resume and go through the same cycle of increasingly threatening letters all over again. I only had someone on the doorstep once and he was polite enough (though looked like a bouncer and was about eight feet wide!). I was nervous but polite and reiterated that I didn't need a licence, and that he was welcome to come in if he had a warrant. He didn't, so he went away and never returned.
I read somewhere online that a Freedom of Information request showed that the number of actual warrants granted to TV Licensing was incredibly low - only just in double figures per year! They have to have very strong, compelling evidence to persuade a judge that a warrant is needed. They can't even walk up to your door if you write to them and tell them you withdraw consent for this.
99.99% of people who are prosecuted for this are vulnerable people who are not knowledgeable or intimidated enough to a) open the door and let someone in with no warrant and b) intimidated into 'confessing' to a crime and signing a statement confirming this on the spot. Many are conned into this in the belief that they are just signing up to pay from now on (which they agree to to get rid of the officer sometimes) and are then shocked when they receive a court summons.
It really gets my goat how they get away with scaring people who have no need of a licence - the language they use in their letters is appalling.
Students can actually watch TV without a separate licence - but only if they are watching on a device which is not plugged in - and their parents address has a licence. Very odd rule!
If they were serious about catching people using iPlayer without a licence, you'd think you'd have to input your licence number to log in. But no - you'll just have to give name, address and email info to log in when it changes shortly. That seems strange to me - what is to stop people just putting someone else's details in who they know have a licence? And don't get me started on 'Detector Vans'...