London is not the centre of the universe and losing an air link to Gatwick is not the end of the world. As others have stated, train times to the capital from the South West are between 3 and 4 hours and the Gatwick service offered little to no connectivity.
The priority should be gaining another hub feeding service, which offers proper connections on the one booking etc. KLM to Amsterdam would be the most obvious possibility IMO.
Also, Aer Lingus regional to Exeter? I don't think its been mentioned before but I don't see any reason why not. Aer Lingus regional serve, or have served far smaller cities in the UK recently.