Virgin will use the existing BT landline to your new house. They will simply notify BT that they're "taking it on". Having said that, if you do have an issue with the landline, it will be a BT guy that comes out and fixes it.
If you have cable, Virgin internet / phone is good, but if you have the landline broadband, it seems pretty rubbish, I'd avoid it if you want internet – ours is too slow to watch iPlayer in the evenings, because they over-sell it and there are too many people on their thing. When we tried to get them to fix it, they basically said that they don't consider what is a 7000kbps line to the exchange (that's what we get when it is not the evening) to be in any way faulty as long as it is getting more than 150kbps. On a weekend, it sometimes can't even play radio on iPlayer, let alone play video.
Looking at traceroutes etc. it slows down somewhere around Manchester where the virgin network goes off to somewhere else, I imagine this is just because virgin oversell the amount of bandwidth they have massively, so it probably happens everywhere in the country.
Basically, I would just recommend going with someone else, even BT was better than this (and I totally wouldn't recommend BT either).
Joe