Sheffield cf MSE.
Yes LGW is 81 miles from MSE. However, Manston's declared catchment population do not all live on the airfield, they live throughout Kent and beyond, almost all of them to the west, so that mileage is immaterial.
The London airports definitely impact on KIA catchment.(I should know. I use LHR all the time.)
According to the Sheffield thread, Manchester impacted on them in a similar way.
The runway length is different, but as long as it can launch a servicable passenger jet, the average punter doesn't care. (Some members of my family did use this airport, and declared it perfectly OK)
I have no doubt the differences outweigh the similarities, but the similarities may be of interest to future aviation historians. Perhap if only beacause it could be said of both airports (to quote Andy)-
"By rights, it should have worked".