The train takes the best part of an hour just to get from Plymouth to Exeter. Mainly, of course, due to the circuitus (whilst scenic) route that Isambard Kingdom Brunel was forced to choose due to Devon's topography.
That additional hour is why air services from Devon's 2nd city to London have tended to work whilst those from Exeter have been, and always will be, unsuccesful.
If you then factor in the rail jouney time from the west of Cornwall just to get as far as Plymouth one can see that a London service from Newquay should be viable.