I was searching google for a solution to this as I am facing the same situation and found this useful thread....
I have just had virgin media installed yesterday and am also keen to leave all of my existing BT wiring just as it is in case I move back to BT at some point or if some other ADSL service becomes better.
I also have and extension that is physically wired directly into the back of the BT socket leading upstairs where I needed the line for Sky Multiroom.
I was determined not to have to get the wirecutters out but finding the cable I thought I needed did not seem possible. I was looking for a BT to BT connection and just connect virgin directly from it's socket into the BT socket.
This is what I have come up with as a solution...
1. Get 2 x BT to RJ45 Adapters:
http://www.run-it-direct.co.uk/bttorj45paypal.html
http://www.run-it-direct.co.uk/rj45tobt1user.pdf
2. Next Get a standard RJ45 to RJ45 coupler:
http://www.amazon.co.uk/Skytronic-RJ...4194638&sr=8-4
3. Plug each end (i.e. the BT ends) into VM one end and BT on the other.
Yeah OK, so it's a bit elaborate but it means that you can leave everything just as it is and you can just get BT to re-activate the BT socket if required in the future.
It should also mean that all of your extensions can stay as they are.
Can anyone see any technical issues with this as I haven't tried it out yet?