David, the fact that there will be 4 daily (well established) flights withdrawn from the BFS London market when Easyjet move the Luton route to BHD, the usual 20k passengers per month that used this route will use an alternative service, some may not want to go to BHD and some may only want to fly to Luton. Luton in its own rite is popular for certain parts of north London and the surrounding areas.
However, its a bit like when Aer Lingus arrived on BFS LHR and they said they were after bmi's traffic, the outcome is that Easyjet have lost significant business from BFS yes Bmi have lost some from City but Ryanair has been there, so its not always easy to say who effects who. All of the flights on Belfast London are competiting for the passengers.
The choice of airport in Belfast or indeed London will be down to
for some to a greater or lesser extent;
Price
Location of desired airport
Do they want to connect to another etc
Business Traveller
Leisure Traveller etc
From either side of the journey the other thing is what do the passengers consider to be their local airport?
If you live near BHD you may only check choice of flights ex BHD and VV
and in London the same rational may apply.
Overall there is less capacity over the winter when compared to LY, as easyjet have cut back their schedule (with LTN move), Bmi have cut by 1 a day and its often more 319s than before (less seats). While demand is expected to be down there still will be less capacity.
I still will be surprised if Ryanair dont see Easyjets move as an opportunity to get in somewhere in the Belfast Luton market and make some noise. If that happens well then we will have an entirely different story.
(I think Ryanair are doing 4 daily on Stansted for the winter)