I assume you have been in contact with the CEO office:
neil.berkett@virginmedia.co.uk ?
Problem with getting a future reduction in billing, even with the phone line moved to BT so you don't need a VM one, is that broadband and TV services are usually sold with the phone line as a required add on, albeit at an extra cost for that phone line. If you don't take the phone line, a broadband / TV service may not be available, or the cost of that service may rise disproportionately. It's quite likely that similar telcos' like BT have similar phone line rental requirements if you want other services from them.