TrueBlue,
You've always held this theory and you might just be right. I've used easyJet on LGW a few times in the last few weeks. Flights full on early out and late back, while interestingly Aer Lingus still selling seats at reasonably low prices 2 days before flights and not full. The easyJet brand is going from strength to strength here.
Ryanair to my mind have done next to no marketing for the LGW BFS route, perhaps they don't think they need to. The last foray into Belfast the marketing splash was huge..