Air Asia X
People seem to have a short term memory... Monarch use to operate their A330s non-stop from LTN to Orlando pre 9/11 attacks! So Air Asia X operating from Luton's "Short Runway" is feasiable, especially when all their flights between London and Malaysia will operate via Dubai which is about a 6-7hr hop...
Air Asia X will drive the hardest bargains, for airport fees and as long as the airport they choose is capable of handling aircraft of this size they will not be picky... For example in Australia they have choosen Avalon as their airport to serve Melbourne and is about 55km from the city and hardly used by by pasenger flights other than a few Jetstar flights.
Been realistic though, I think Stansted will be their choice... But LTN should not yet be written off, if they management offered them a good deal. Airports that see few passengers these days like Biggin Hill, Southend or Manston should not be discounted!