I don't know that Tiverton Parkway is a strange omission - it will have an hourly London service and presumably GWR has plenty of evidence from ticket sales as to the overall level of demand to/from Reading and London and thinks it doesn't justify more than an hourly call. Pathing may also put the semi-fasts close behind or just ahead of an XC service or GWR Bristol-Devon/Cornwall train (both running in a regular hourly path, as opposed to every couple of hours) as well, in which case the odds of picking up/putting down any local traffic would probably be limited.
I think there will be some 387 extras in the peaks between Paddington and Newbury as well as the Bedwyn IETs, plus 387s on the Reading-Newbury stoppers. I gather the number of weekday Newbury-London through services both ways will go up from about 40 now to 50-odd, made up of IETs on Bedwyns and the Exeter semi-fasts, plus some 387s.
It would probably need five 800s, instead of three, to provide an hourly service out to Westbury and I doubt there would be be the day-round demand to justify an hourly London service there, never mind Pewsey.
How could you use such a service, as there would be no connections to/from Exeter at Westbury, other than the semi-fasts, which will be calling at Bedwyn and Hungerford anyway?