BT have nothing to do with thw virgin line, they are completely separate networks – so if you don't already have cable, you may have to pay a connection fee. Since you have cable broadband though you should be ok
We have both – cable for home, BT for Mrs NBT's home business. The cable broadband has been fine, and you do get free cable tv if you take broadband and phone which is useful. The phone service is fine as regards talking to people – it falls down a little over BT when you consider the 1471 and 1571 services, as they're frankly not as good: the virgin line doesn't recognise as many incoming numbers as the BT line and will only tell you about the last number it recognised, so if you don't get to the phone in time, 1471 might tell you you were last called 4 days ago, kind of thing
the 1571 service has a very limited lifespan for saving messages, and does stupid things like deleting a message if you listen and then hang up without explicitly choosing save
they're minor niggles though so maybe not too much to worry about