Not actually driving to the airport makes the extra few miles past Kettering possible! However I'd still say it's not in Cambridge's catchment because it's almost as close to an airport that will always be a bigger draw.
I'd say Cambridge's catchment is Cambridge, villages west of Cambridge before the A1, villages north of Cambridge until before Peterborough and Kings Lynn, villages east of Cambridge up to Bury, and perhaps a few miles south of Cambridge at the very most.
I don't see this as an insurmountably massive inherent problem because there is so much within that area, and a lot of people travelling back and forward from abroad to get to it.
And I was surprised that people I met on Darwin flights had come from Norwich as the airport development fee there upset them, so the catchment might be bigger than I think!