I agree with gilesdavies. For the amount of money it would cost in building the extra length I can’t see them getting their money back in the short to medium term or even the long term. As it is most aircraft now depart from the intersections and don’t even use the full 2160m.
Luton does not have the space to increase the size of the cargo stands so the cargo airlines would not use a longer runway very often and I can’t see that much if any demand from the airlines to operate long haul passenger services as there isn’t that much demand from Stansted.