It's not absolutely no reason, instead of using call centre i was able to use live chat to book an engineer visit. I pay for 130mbps and to get 30mbps which gives me an unplayable experience is clearly something that shouldn't happen. Especially when others with different ISPs don't have this issue but many with BT do.
Could it be that when using your VPN that the route taken to the Geforce Now server(s) might be totally different and avoids a congestion?
I say that with very little experience of that side of things but just offering up a possible explanation.
Looks like the VPN is taking a more optimal route... You are getting timeouts on the non-vpn tracert but no timeouts on the other... the first timeout looks like its coming from your public IP going out to the internet, so might be your router/sh2, and the second timeout seems to not be on BT's infrastructure.
I would have a look at doing a reset of the router/sh2 and go from there.