Not sure if it's an option, but everyone seems to forget about Gatwick these days. Gatwick now comes into play on Star thanks to TAP and mainly US Airways flights.
I've used the train from Gatwick to get to Plymouth in about 3h 00m in the past (so what's that, about 2 and a half hours to Exeter?) From memory there's a train about 9.15 Gatwick to Reading and a very quick (10 mins) connection there onto a Great Western to Exeter train. The 9.15 departure connects well from most Nth American inbound flights, including US from both PHL and CLT, both with direct connections inbound from CHO.

Dunno about anyone else, but I prefer Gatwick over Thiefrow any day, but I guess that also depends on destination in London or elsewhere.