In my experience, sounds travels through the floors a lot. It’s especially bad in Victorian houses and with wooden floors obviously.
I think you might have limited success tackling walls only.
You can install stuff in the floor too, but this is probably labour intensive and costly.
We have one soundproofed wall internally and it’s not especially impressive.
It might be a good idea to mention this to the neighbour in case they can move the tv in their flat a bit. Got never know, maybe their layout will allow for it. As long as you are reasonable and friendly, might be worth a try.