3 hours 30 minutes from Plymouth to London on a Class 166 sounds fairly horrendous (Yes, I know, it's similar from Hereford and thats equally unacceptable) to me. We should be using DMU's like this LESS on long distance services not more.
It is more likely to be over 4 hours, however given that most of the direct intercity services would be at least 30 minutes faster then it is unlikely that there would be many people who would use it end to end. However there would be a number of people who would be willing to travel over shorter but still fairly long distances (as suggested Exeter to Reading) as it would get them where they needed in a better timeframe.
For instance there is a service which currently calls at (which is a similar calling pattern to what was being proposed, although it calls 10 stations when there are only 11 between Exeter and Newbury and it was suggested that some stops would only be served every 2 hours):
11:54 Exeter St Davids
12:09 Tiverton Parkway
12:24 Taunton
12:45 Castle Cary
13:05 Westbury
13:22 Pewsey
13:33 Bedwyn
13:40 Hungerford
13:50 Newbury Station
13:57 Thatcham
14:06 Theale
14:17 Reading Station
Which takes 2:22, compared with the fastest travel time between Exeter and Reading of about 1:30 (although most services are closer to 1:45). Yes if the next train to Reading from Exeter only takes 90 min you're going to wait for that over a train which takes 140min (unless it's over 50 minutes later). However if the next train takes 105min and isn't for another 45min then you may take the slower train as it will get you there 10 minutes earlier.
If you look at service patterns and time from just one station closer (Tiverton Parkway) and the current service patten can mean that the gap between sub 2 hour trains to Reading is 2 hours and the semi fasts look a lot more promising. Even as far up as Castle Cary it can be a 3 hour wait between direct services (the indirect services take over 2 hours).
Whilst by the time you get to Westbury (where the travel time difference to Reading will be much lower, even on the above service pattern it's 20 minutes) where the gap between direct services can easily be 2 hours and the indirect services take at least 30 minutes longer than the direct then the semi fasts really come into their own.
Not only on direct services, but the semi fasts could make Weymouth to Reading at least comparable in terms of timings as using SWT's and XC/GW going via Southampton, depending on change times it could even give SWT's a run for their money for people going to London from Weymouth.
