As I'm going to the USA I've disabled voicemail as this network has the practice of charging you for the mere act of someone leaving a message - over which you have no control - as well as then making roaming charges to receive the message. That's bad enough - but my attempt to have calls forwarded to another number (also UK) where they could be handled failed on my phone and I was then advised that the Virgin mobile system won't do that. As other networks do it's clearly technically possible and I don't see why Virgin won't do it - unless it is to make money out of unsuspecting voicemail users.
Sadly you are correct in that they don't supply that service, I suspect it's more down to not being willing to pick up a tab for getting the service added to their contract, as they can't be bothered with obtaining and supplying a 4G service, I'd expect hell to freeze over before call forwarding is enabled. On the day it is then the cost might make it not worth the hassle.