Silver,
he south coast line (the way I went last time) is slow and tortuous, and often stopped by rough seas.
You are talking about the South Devon Sea Wall around Dawlish Warren - that is beyond Exeter, plenty of dramatic photos of that section exist, it is perhaps the rail equivalent of SXM or Kai Tak. Anyone, back to airports:
I put it to you that the real reason that there is no Flybe turboprop to LHR is that LHR is full, and there are no slots.
No, not for the very short hops - APD, fuel costs, the whole wastefulness of such a short rotation, - that is why none of the airports I mentioned have service to any other London airport either.
So I put it back to you - even if it might be
desirable to have service through LHR, from CAX, MME, LDY and so on - they are all a bit further away, the huge investment which would be needed in a new airport, spending £2 on relocation for every £1 spent on expansion, combined with the costs of an island location - just aren't worthwhile.