Hi jimbo, regarding your querie about interior wiring, I assume you are refering to extensions wired from the existing Virgin master socket?
If you do have extensions wired (directly) from Virgin's socket (i.e. not just plugged in, but wired off the back) then you will lose service on these when the service is ported accross to BT's line, as this is a different master socket.
However, you may find an Openreach installer/engineer visits on behalf of Sky. If this is the case you just need to ask him to fit BT/Sky?openreach NTE next to Virgin's NTE (master) and piggyback the extensions. Ask nicely and he'll do it no problem