the train may not be quick, but it is much more frequent
I'd question that assertion also, Flybe currently fly 6 times a day SOU-EDI (in both directions), 5 times a day EDI-LCY, 7 times a day EDI-BHX and "only" 4 times a day EDI-MAN, and (impressively) between MAN-EXT.
None of those are indeed very quick by train, but are almost comparable for frequency.