Virgin have(had?) two speed problems:
Contention - this used to be awful, haven't been with them for a while now, but it was a serious problem. I wasn't in a large city either...
Upload speed - I used to saturate my upload trivially, IIRC it was <10% of my download rate. That's extraordinarily poor. And with the vast increase in 'sharing' and 'using someone else's computer' this is more and more important.
The download was great - their customer service when presented with logs indicating a failed coax in the street cabinet was appalling. The failure manifested between 9:30 and 10 at night until 6:30-7 in the morning (temperature related) and they consistently sent engineers out at midday - who said there wasn't a problem.
Eventually I got an actual network engineer and we looked at the houses with issues, and decided where the fault had to be. We were right - and it took 5 minutes to fix it.