As UKtravelbear says, Virgin won't normally upgrade anyone to a higher cabin just because there is space. There may however be some paid upgrades to UC that clear some seats in PE for paid upgrades at the airport.

As I think I mentioned in the original thread about paid upgrade options, when flying from an outstation like Seattle, a lot depends on whether they have been given the green light to sell seats. Unlike London or Manchester, where they are set up to grab some extra revenue when space allows, Seattle may not be able to due to seats being out of service due to issues on the way out, or not enough catering for a higher cabin.

You may get a good offer though is things align. Do let us know how you get on and I hope you have a good flight over to London.