Hi, We have had an extension to our house and I have run 2no coax cables back to the hub point (I'm told the coax is VM compliant). I have run 2no cables to give us the flexibility to have the tv in different sides of the extension, so only one will be used at any one time, we'll be looking at upgrading to multiroom.
its been known for VM to use pre installed cables but its rare - i would work on the assumption they would not - what the TV installer says is of no interest to VM - you will have to ague the point on the day and hope you get a tech prepared to test them and use them
In follow up to the advice given by japitts. If you're looking to get multi-room, the cost could depend on available offers at the time. Your best best would be to call our team on 0345 454 1111 or 150 from our mobile or landline service.