you power levels are pretty much perfect which means it is either an area fault or you have got a dicky modem. The fact that they have already admitted there is/was an area fault makes me think they are still having a problem. See what this says:
https://my.virginmedia.com/service-status/details
Failing that, ring tech support again and tell them you want an engineer to come out and then at least you'll have an english person to talk to. You can explain the problem to him and he can look into the area fault and make sure it has been fully resolved.