It's the DSL providers like BT, TalkTalk, Sky, etc who force you to have a landline as their broadband must be delivered over the phone wire coming into your property. Although VM doesn't force you to have their landline a lot of deals are cheaper when it's bundled with broadband/TV.
when the VoIP goes public it should work out cheaper
Virgin already has closed circuit VoIP for new customers. The phone plugs into the internet hub, but this only dispenses with the copper twisted pair between the customer & Virgins Local exchange. The price you pay is the same as if you were still using the copper pair.
As a Very Insightful Person, I'm here to share my knowledge. I don't work for Virgin Media.