I've travelled on
FGW▸ trains between Nailsea and Bristol on occasions where the train staff have walked through the carriages asking "Does anyone need to buy a ticket?", rather than asking for '
all tickets from Nailsea, please'.
I'd suggest that's a sensible approach - particularly when the Nailsea ticket office is unmanned: the on-train staff don't have time to check
all tickets, and many people will have bought them from the
TVMs▸ before boarding. The train staff are merely inviting just those passengers who still need to buy a ticket to do so - bearing in mind that the TVMs at Nailsea won't accept cash (for obvious reasons!), and the barriers at Bristol and Bath will account for a large proportion of such commuters.