Thats slower than Paddington semi-fasts. With an hourly semi-fast to padd FGW can effectively compete on both fronts.That's the only way I think it would work,would probably lead to some strange pricing anomalies though. On the fasts it would probably have to end up being cheaper to travel further. I do think there should be a semi-fast service from Exeter to London, but I don't think its purpose should be to take lots of the passengers off the fast services. It should be to serve the smaller stations along the Berks and Hants line.
There is - it runs hourly to Waterloo from Exeter St Davids - first train 9.26am, last through train 7.26pm. Fares are much more competitive than the Paddington route.
Plus SWT only serve exeter; the semi fast to padd would also serve tiverton and taunton.