Well my wife did indeed just call the TV licencing people to let them know we no longer need a licence. It went pretty much as I expected. Part way through they asked to speak to me as I'm the licence holder. It was like being interrogated and being treated like a potential criminal. They wanted to know what we were watching such as streaming services. I refused to tell them and just kept repeating that we don't watch live TV and don't need a licence. But she didn't seem able to not continue with her script. Then they "wanted to make me aware" that they may send an "officer" around at any time to check. At that point I told them they had no right to enter the house and I was not agreeing to them doing so. The agent then said she would have to transfer me to another department. I've no idea what that was about but I assume it was because we weren't answering her questions. So my wife grabbed the phone and told them in no uncertain terms they were treating us like criminals and we are fed up funding their sex scandals
We put the phone down at that point.
I did warn my wife beforehand not to bother calling them as this is how it would go. But she had said she wanted to do the right thing. I was prepared for it as I've been watching this situation unfold for a few years (thanks ChillieConCarne). But she genuinely thought she was doing a nice thing trying to let them know. Now she's fuming as she feels she's been treated as someone trying to dodge it.
I've told her to simply bin any letters and to shut the door on anyone coming round. I've also got her to uninstall iplayer from her phone so she can't accidentally start it. I'll warn the kids to do the same too.