Part of the problem with Oxford / Cotswold services is the demand to Oxford outstripping the capacity of a 5-car 80x and the Cotswold bit of the journey not needing more than 5 cars. However, there isn't an easy answer to that with the lack of timetable capacity at Oxford to split trains.
Probably the only route on GWR where splitting would indeed be sensible. But partly due to a lack of electrification, P3/4 do indeed get extensively used with shunts. You'd ideally want a south facing terminating platform.