I'm on there 50Mb service right now. It's excellent. Uncapped, 50Mb speeds at all times of the day (even peak times). The upload speed could be better (only 1.5Mb) but Virgin are currently trialing 10Mb upload on 50Mb connections in certain areas and they issued a press release ages ago mentioning the 10Mb tests and that it could become the norm for high speed connections on their network or an extra service option.
I'm confident they could deliver 100Mb+ connections based on my experience with 50Mb. I know a bunch of other people on the same service and we all agree that it's stable and fast at all times of the day. One thing to keep in mind is that Virgins 10Mb and 20Mb services use DOCSIS 1.1/2.0 while the 50Mb and above use DOCSIS3.0. And as shown in the article their DOCSIS3.0 infrastructure is way under-subscribed at just 70,000 customers. They have millions on the DOCSIS1.1/2.0 infrastructure.