I was originally with Telewest, and, yes, on the odd occasions that I do log in to check mail, I do have to enter the provider. I must admit I have never really used the mail facilities with Telewest/Virgin, partly because I have other established email accounts and partly because I could never remember my logon id and was too idle to go looking for the bit of paper with it written on.
If you think about it, getting all the NTL and Telewest customers to change their email addresses to the Virgin format (just think of how many people would have to be told about the new addresses - and I'm not just talking about the customers !) would cause a lot of trouble for people. It's much easier to maintain the original addresses (and probably servers as well) and direct customers to the right place by getting people to select from a short list of original providers, albeit at the cost of a minor inconvenienc to the customers.