I find it hard to believe that there isn't a market for an arrival in Exeter from London in time for a business meeting or conference at 10am. Things do happen in provincial cities. Exeter is the hq of South West Arts, for example.
Based on past experience when services improve, I think FGW▸ might be surprised at the number of passengers an early service to Devon would attract. After all there hasn't been one since before the "railway renaissance" began.
and the met office, theres also the holiday traffic.....back in the day we used to catch the early train down here before we became residents, who wants to spend the first day of your holiday traveling? i have great memorys of arriving at exmouth at 6am after leaving stockport at 1am