Seats can be requested in all classes. However, there is only a certain percentage of seats that Virgin will allow to requested (may be 80% or may be considerably less - I forget). Once that percentage has been reached, no more seat requested will be taken before check-in.
Even if you "request" seats you may not get what is requested (they may be given away at check-in). So the best strategy is to check in as early as possible.
Sorry can't be more helpful.
Boo