You only have to look at the traffic figures at many UK airports over the last couple of months to see that passenger numbers seem to be stabilising, albeit only for a short while. There is no point in expanding your infrasctructure unless it is going to be used to its full effect.
If that is the case I wonder why then that BAA are spending million$$ at Stansted???
I guess when they get their second runway they would never dream of trying to tempt airlines flying into Luton that they should use Stansted.