It's very unusual for virgin to lay any new cables - you must be fortunate
yes it is quite possible for your connection to suffer everytime it rains - problem is finding where rain is getting into connection. You need to get an engineer visit but problem is getting visit when raining
Did you try the quiet line test? If there is a water problem causing your internet problem then I would expect noise on your line
The only thing which you have not ruled out is a faulty filter and I would try a new filter before I got an engineer visit just in case that is the problem as it would be an expensive new filter at £129.99
There are other posts on the forum where the problem was water ingress just as you describe but it was also causing problems with the phone as well.
there are also posts on the infinity board just like yours regarding the street cabinet and why it was one of t.he ones not upgraded but openreach do not respond any longer to the previous nga enquiries