Sorry to be the bearer of bad news... If I were you (just signed up with the intention of playing online games and with a BT line already installed) I'd definitely be looking to cancel VM.
Latency on VMs internet is (as you have seen yourself) much worse on their massively over subscribed service. VM do not accept that there is anything wrong at all - as far as they are concerned if you can view a web page everything is working as it should.
Since VM do not even acknowledge the issue we can only assume that absolutely no action is taking place behind the scenes to try to fix this. The only hope for those of us trapped in contracts is that the load will reduce as lockdown eases (but that could be a while yet).
Save yourself a lot of money and aggravation, cancel while you can.