These small airports should be excluded from APD, I would add Teesside, Humberside, Carlisle,Doncaster & Exeter, they serve sizeable local communities and would reduce road traffic and improve business links, CWL & PIK are a matter for their respective devolved assembly's although the business case for PIK is IMHO with GLA up the road flawed to say the least.
Although Exeter & Plymouth are close-ish the problem is with Exeter being on the East side of Exeter and road down to Plymouth is a bit of a night mare in Summer with traffic and although the road is mostly dual carriage it doesn't feel safe especially at night