am i allowed to have a family member call up virgin media on my behalf with me there to cancel my contract for me if i give permission over the phone for that person to do so?
Hypothetically, of course, VM can have no idea who is actually calling them, if your family member claims to be you and can provide all of the necessary information then they really have no option but to accept it.
So if it really was you calling what would VM ask to ‘prove’ it was you? There is a telephone ‘password’ they should ask for, do you have one and if so do you know it so you can let your friend know? Otherwise, it’s things like amount on the bill each month, the exact date that the bill is paid etc. As long as you know all of these and the person calling on your behalf knows them as well, then I can’t see there being an issue.