A london route to exeter wouldnt work, competition with rail means it cannot be profitable, Flybe have already done studies into an Exeter-London route.
Here is what Plymouth City Council has had to say, some interesting tibits;
The Chamber is now calling for an independent study into the future of air travel from Plymouth to cover areas such as the feasibility of a runway extension and using new technology to shorten the distance needed to take off and land at the airport – which has a short runway that cannot be used by many types of aircraft.
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A spokesman for Plymouth City Council said the news was "disappointing" and that it would be discussing the implications of this decision with the airline.