You just file RVSM on the Flightplan. Not sure why ATC would contest that?
Because there have been quite a few filing as RVSM approved when they aren't.
I believe they have a register of those that are approved and perhaps the Virgin one had not yet been added or not yet approved.
Same applies to ADSB - though that's a bit more obvious - the aircraft doesn't show up as ADSB track when it leaves radar coverage.