You need a TV licence if you watch or record TV as it's broadcasted (or shortly afterwards) or if you use iPlayer.

You go on a website, enter your address, confirm like 3 times that no, you don't use your TV to watch live programmes and that's it, you won't get another letter for a couple years at which point you just need to re-confirm that again.

You need a licence if you receive live tv as it's broadcast, or if you watch BBC iPlayer (a video on demand "catch up" service).

You don't need a licence if you don't watch live tv as it's broadcast.