LHR-Exeter on National Express coach is more than 3 hours, try 5 or 6 !! Charters can be arranged from LHR to the West Country on something like a Chieftan or a Cessna 406 for example subject to slots - either that a charter be done out of Northolt or Fairoaks, other smaller airfields in close prox to LHR. PM me if your wish!