Yes. It is here. http://my.virginmedia.com/traffic-management/traffic-management-policy-thresholds.html
It is only on upload though. Stupid really. Someone thrashing 12mbit upload 24/7 is going to cause much less issues than someone thrashing 200mbit download constantly.
RE VM, they do sometimes have congestion at peak times but your performance should and probably will exceed BTs. Only issues really arise when your road has a much of heavy users all using the same provider. I was with VM for 15 years and everything was great the entire time. Only moved away so I could get more upstream bandwidth.
12Mbit is a substantial amount of the upload capacity though. I can't remember the actual numbers, but the nature of DOCSIS is that the upstream capacity is nowhere near equal with the downstream (I want to say maybe 10% as a rough guide). That's why they don't want you maxing it out for too long, and why they can offer significant jumps in downstream speed but hardly anything on the up (and also why Openreach really couldn't care, because they don't have the same constraints - though why they only offer 20Mbit upload to the lucky few who can get FTTH I don't know).
Of course neither direction is bulletproof - with the new 200Mbit speed that Virgin plan to offer, they only need to have a few people maxing that out for demand to exceed supply and for peak time congestion to result. So much for "vivid optical fibre" (that has GOT to be a potential ASA case, surely? That implies FTTH when it is no such thing).
At least Openreach doesn't appear to be suffering from backhaul issues with their FTTC cabinets, yet (as someone who spent ages trying to get BT Wholesale through a competent ISP to fix it on 20CN ADSL on more than once occasion, I'm very glad), and it's just line length/RFI/crosstalk to worry about
