I based that on only a rough application of the speed limit
01475,
In my 1000cc ''thundering fart'' Higham Ferrers to Hampton-in-Arden rail station was 59 miles with the vast majority of the route being dual carriageways.
I would regularly complete the journey in less than the hour to then park my car at Hampton-in-Arden rail station for free to take the 3 minute train journey in to BHX airport for something like 80p each way.
And Higham Ferrers most definitely falls within Cambridge's catchment area were there a realistic functioning airport at Cambridge.
As mart901 pointed out London airports can be a pain in the butt to get to and from Higham Ferrers LTN was OK to drive to/from, just an unpleasant experience when one arrived there
, to get to/from STN one would be driving past Cambridge and if any journey involved the M25 it wouldn't possibly be quicker than driving to/from BHX or indeed EMA.