It just proves how completely meaningless that Which? report actually was, I am not saying that the results they got were wrong - just that they were obviously not measuring the things that really matter.
I suspect if Which? tried to measure "the countries favourite Motorways" the M1, M6, M4, M25 (etc) would obviously come bottom.
PS is all this growth at Luton in the existing peak hours or is it that other, previously quieter, parts of the day are getting busier? - that makes a big difference to wether an airport is "full" or not.