I've had the same thing for about 2 months. Getting full signal but data is really poor. I called Virgin help, who then told me to reset my data setting on phone. this in turn killed all data on my phone as it wouldn't sync to Virgin and needed a new sim. After breaking my sim they told me they were doing mast upgrades in my area which may have impact (although I asked them that directly at the start of the whole saga and they said no). When I asked about billing - Paying for 4G and not getting it then no working sim, I was told my bills would be adjusted to reflected reduced\loss of service. I'm yet to see any reduction in bill. It's even more frustrating as home WiFi isn't reliable as we've just gone to PlusNet (phone is connected to WiFi but no data transfer) but that's a forum for another site. I'm not impressed with Virgin and already moved away broadband, cancelled land line and TV. Unless it gets sorted soon I'll also be moving the households 3 mobile accounts too.
I've just cancelled and got my PAC code, 2 months of no data service in central London, nothing visibly being done to fix it, no updates from Customer Services when promised, no ETAs for fixes, no account credits materialising when promised by Customer Services. The whole thing is a complete shambles and the network not fit for purpose.
0.01Mbs download speeds in the heart of the City of London Financial District for 2 months. If it takes more than 2 months to identify the issue and fix, then that's a good enough reason to leave in itself.