A few posters have alluded that a direct Barnstaple - London service might be on a few wishlists, but the only suggestions I have seen are as an extension of the SWR London Waterloo - Exeter St David's service. If I were in Barnstaple I would want something better for my London service than a class 158 for a 4-hour trundle around the Devon/Somerset/Dorset/Wiltshire borders.
I am surprised that no-one has come up with this suggestion, so here goes:
In the next couple of years, GWR will have nice shiny class 802 bi-modes racing down to the West Country.
I suggest that two five-car units running in tandem provide one or two daily services between London Paddington and Exeter St David's, calling at Reading, Taunton and Tiverton Parkway.
At Exeter St David's the two units are divided. The leading unit continues on to Paignton calling at Dawlish, Teignmouth, Newton Abbot, Torquay and Paignton. The rear unit reverses out and heads for Barnstaple - I don't know that line well enough to suggest any intermediate stops other than Crediton.
Likewise two Up services doing the same in reverse.
In terms of the Barnstaple branch, I don't know how practical this would be. Could an IEP even operate on the branch? I would also imagine there to be pathing issues, presumably it would have to take the path of an existing local service, and this might have implications for the service at intermediate stops if it were not possible or desirable to stop the IEP there. I also don't know if there would be a safety issue that would prevent two services being joined together at Exeter St Davids if they were arriving from opposite directions.