BE would pick up new customers if they went to Stansted...
Customers who want to travel to areas in north/east London where Stansted is more convenient for example.
Customers who had previously used routes such as BHD, NCL and NQY from Stansted when served by FR and EZY.
Customers who are currently having to do what "Haven't a clue" is concerned about, having to travel to a less convenient airport because they haven't the choice.
BE would have more of an opportunity to build a stronger customer base by serving a currently underserved area of the market, instead of fighting for the same customers from the same areas as EZY and BA.