Just wanna say that sounds majorly shitty. I'm assuming you guys already tried solutions including port forwarding, switching to wired connection, etc.?
The fact that it only affects you two connecting directly to each other makes me wonder what it could be. The problem sounds very specific, and may necessitate calling your ISP. Of course, I don't know how things work in the UK so I'm not sure how much mileage you'll get from that. Hope you guys get the help you need.
But you're old, you shouldn't be sitting in beanbag chairs like some hip youngster. D:
Seriously though, I want a beanbag chair now because they were always the shit when I was a toddler and damn were they comfy to sit in. They could probably do something to help me relax in tense Smash situations, too.