It is good to try to establish how folks immediately close to you find the service. You now have one such view and this is as I would expect - nothing highly unusual about either the day time or evening speeds in a dense urban location.
You can get another view at the Virgin Media Support Newsgroup - virgin media support broadband cable (You will need to know how to access newsgroups -described on the
www.virginmedia.com website)
There has been a small but growing number of complaints related to slow speeds. However, I cannot re-call any issues in West London. Virgin Media has done a good job in improving the level of network service for most of its users in most locations. But, beware: there are some very poor spots such as my immediate area in North London and also in East London and the service is often so poor that it makes our blood boil!
You need to understand the traffic management rules also as covered in the replies.
Given the above comments about the actual impact of the exchange move, my guess is that going with Virgin Media is likely to be a better option, particularly since the 20 and to be introduced 50 mbs services will be working on a different signaling system into next year. But also be prepared for some hiccups.