You should be able to purchase an upgrade from coach to PE at check-in, if there is space available in PE. As far as price is concerned, these paid-for upgrades can be cheaper that pre-paid upgrades depending on how busy the flight is.
I appreciate that it may not be possible. In fact I am assuming it won't be possible! Its good to know who to ask though.
I'm really looking forward to it. We flew Virgin Atlantic in 1993 but since have only been to Europe on charters. VA is positively premium compared to those anyway (or it was in '93!)
Compared to European charters VS is 7th Heaven (apologies to Air Jamaica), I've just done LGW-TFS-LGW on Air2000 we paid extra for exit seats but, oh to be on a VS flight!