Any issues with Trellis or fencing extension on top of garden wall ?

I had hoped to add additional trellis / fence as my neighbour has extended their decking. It's a nightmare. Last year I swore I wasn't spending another summer with them head and shoulders over the wall. It's unfortunate that there is no planning required on decking as my privacy is totally compromised. They can see into my kitchen from standing. When they have guests they peer in. In summer if I'm sitting / lying / hanging out washing in garden they can see me and can engage where I don't want too. They were hanging something on their side of wall. And from standing I had them both overlooking me while I was in the garden. I'm so annoyed that it could cost ME to restore my privacy to what it was.

You can go as high as 2m to the rear without the need to seek planning permission.

My understanding is that the limit is a maximum of two metres to both sides and to the rear.
The reason has nothing to do with light/privacy/etc. It's to permit access in the case of an emergency should standard access not be an option.
For example, if there was a house fire the fire brigade may need to access your burning house via your neighbours garden and yours. Hence the height limit and the ban on broken glass, punji sticks, etc. on top of walls.