Virgin, like any other prospective owner, can apply for a six month "reservation" of the name.... but that is very different from formal registration and things may change and cause the "reservation" to be null and void since its not guaranteed !
You can renew your registration reservation every six months for an indefinite period, provided you pay your £150 each time. I can't think of any circumstances that would cause the reservation not to be renewable.
Dave Cummings asked about REGISTRATIONS and he got an answer about REGISTRATIONS !
The CAA differentiates between "registration" (the process of placing an aircraft on the UK civil register) and "registration mark" (the resulting G- mark that is chosen/allocated to the aircraft in question).
However most of us lesser mortals, and clearly that includes the OP, use "registration" to mean the actual G- registration mark involved. Hence the helpful responses from other posters who understood the question.