[Halcyon]

Unless any different to Virginmmedia's policy, you are given the modem for use during your contract with them. It still belongs to them.
Selling it would therfore be illegal.
As has been mentioned it will not work on a Virginmedia connection.

Often they are not that bothered if they go back or not.
Just phone them up and get an address if you want to get rid of it and post it to them.

There is nothing else you can do with it.