Bit weird about the Virgin guy but I can't see the problem with post arriving. When you give your new address to people they don't always get timings exactly right. Some post and a parcel arrived for our buyers before we moved out and I just assumed some kindly friend or relative sent them a house warming present or something and wanted to make sure they got it. And various bits of post arrived for us at the new place, including keys for the holiday cottage we are renting. I had given the owners both addresses as I didn't know when they would post the keys.
Can't see why anyone would mind. You have to be a bit chilled when you move house as lots of it is a bit imprecise.
I think it's cheeky. I wouldn't give anyone the new address until completion and you could just say no to the Virgin Media person because they're not entitled to have anything done on your house and would have to 'undo' it if the sale fell through.
When we recently moved I phoned bt to arrange having my service transferred to new house and the people buying my house had already spoken to bt to arrange things, it made it very complicated and the only option was for me to cancel their order (mad) to allow my order to go ahead. All very annoying... if they had waited till I had arranged moving my service then their order wouldn't have been cancelled. I think people get really worried about being without broadband/internet.