Alloy, had the runway have been extended years ago, then I am sure the airport would have surpassed the 18 mpa figure already!
Really???
While airports like Stansted may have achieved this figure, it is nothing to do with the runway length.
I've just plucked a figure out of thin air, but I would say about 98+% of their traffic could be serviced from a runway the same length as Luton! Especially when you consider both airports are currently operating the same sort of traffic. Low cost carriers which operate 737 and A320 family aircraft, and charter traffic up to 757 size...
Stansted does have a healthy cargo business that attracts MD-11's and 747's, but there is no more than a handful of them a day. The rest being A300's, 737, 757 and 767, which LTN can already serve.
The long haul traffic STN has attracted in the past was Continental with a 757 to Newark, which could easily operate out of Luton. Bristol has a shorter runway that LTN and they use to host a similar service a few years back.
The other long haul traffic, has been and gone and couldn't be sustained, for example Air Asia X service to Kula Lumpa with a an A340-300, and American Airlines to Chicago with a 767-300 (which with some restrictions could operate from LTN).
Luton would have the same issues with Long Haul traffic as Stansted does.
Birmingham is in the process of extending its runway to 9-10,000ft. They are hopeful this will attract further long haul routes. I'd be interested to know who that is, which cannot already utilise their existing 8500ft runway?
Emirates and PIA already operate 777-200/300's from the existing runway. The only candidate I can think of is, Emirates upgrading to the A380. But an extra length runway just for their use, seems a little extravagant.
If other airlines want to operate further distant routes, I would expect these to be opened up with 767/787/A330 which could all operate from the existing runway.