Bit late to the party but you can't book seats until 90 days out. We looked at going PE on the Dreamliner as the saving was about £3,000 compared to virgin PE (family of 6) but there's no way I'd want to wait until 90 days out to get our seats. The idea of being split up fills me with dread!
With VA we got our seats together the second we bought the tickets (11 months out)