Exeter will be a tight one versus the train, in terms of time and cost.
Typical times from Paddington to Exeter with FGW are around 2 to 3 hours, with the fastest that I can see (with one stop at Reading) at 2hrs 2mins.
You need to add the best part of 45mins to that if travelling from The City/Canary Wharf, but it still looks marginal as to whether flying will actually save you any time door to door.
In terms of cost, rail fares can be anything from about £80 (cheapest off-peak return) to £230 (anytime return), and more for First Class, so it's going to be a close-run thing both time and cost-wise.
The trains are always packed though, and flying may "feel" quicker, even if it isn't.