Hi @BobT, thanks a lot for your reply and no problem.
In response to your points:
a) It depends on how it's connected at the moment. If your base unit is connected to the standard telephone socket, then it will instead be directly connected to your Virgin Media Hub/Router. There won't be any cabling that runs anywhere for your main unit, as it will literally be connected with one end in the back of your Hub, and the other end in the back of your unit. (This also answers point c of yours too, so no a cable doesn't really need to be installed anywhere).
b) The performance of the broadband won't be impacted in anyway, so please don't worry about a deteroriating impact on your services this way.
What I'd like to do though, is book the switchover appointment in for you as it's possible that based on your layout, you may need some extension adaptors or additional wiring to ensure your set up works correctly still.
Please let me know if you'd like me to arrange this and if so, I'll pop you a PM asking you to confirm further details - I look forward to your reply.
Many thanks
Tom_W