Maybe I'm wrong, but my understanding wasn't that they couldn't. More that they didn't want to given it may give people the wrong idea since these services will form the Henbury services in time meaning they couldn't simply be extended to Parkway from Filton like now. Basically giving a worse service now to avoid the obvious negative reaction when they'd have to cut it from Parkway again in the future.
I think there'a a bit of truth in both arguments. There's no doubt Parkway struggles at times even with its four platforms and two goods loops, as so much has to get routed through the flat junction immediately west of the station. You don't want to make that worse, especially if those trains are starting/terminating there and taking up valuable platform space whilst they do.
Most of the
BRI» -FIT-BRI stoppers do their reversal from P4 to P3, via a signal BI2052 on the way to Henbury. However, just two per day do so from P2 to P1 (i.e. on the Mains), using a platform at
BPW» , and running
ECS▸ (2H58/59 10:46 and 2H62/63 12:54). So the train in question does not, and had to be switched to P2 on arrival from BRI as there is no route from P4 to BPW.
I think the choice of reversal must depend on small timetable details. The move has to obstruct paths somewhere: it's a question of whether a gap can be found on the Wales/Henbury lines (generally easier) or the Main Lines and in BPW. On Monday, with the line onward to Gloucester closed by flooding, most trains that might go that way had been cancelled. So there was both a lack of conflicting occupied paths on the Mains and a need for connectivity via BPW to be replaced - hence the switch. Don't read too much into it!
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