PNZ, PLY» , PAD» , NTA»
Apparently, anecdotal evidence suggests that Trainline contact their customers if a train is advised as cancelled - and this happens even if the train is only 'part cancelled', but not directly affecting their customer's journey [for instance, if a PNZ to PAD service is cancelled between PNZ and PLY only, a customer joining at NTA is still contacted advising that their train has been cancelled].
If this is the case, what should happen if the customer rearranges their plans and turns up for 'the next available service' only to find that their original train has actually been and gone? Assuming they had an Advance ticket, should they lose their money or should they be allowed to catch the next available service without charge?
I guess it really depends how much you care about your customers and the future of your business.
If you do, and want to demonstrate empathy for their situation given that the train they'd booked had been amended and (for reasons outside of their control) they'd been given incomplete information by their ticket vendor through no fault of their own, it's the latter option, together with an apology for delay/inconvenience.
If you want to fold arms, be a jobsworth and rely on a rulebook to back you up, it's the former option, but bear in mind that next time they'll probably take the car, and tell all their friends about the way they were treated too.
If the railways are sincere about improving customer service/experience and attracting leisure travellers, I think the answer is obvious -
GWR▸ can have the discussion with Trainline themselves - and perhaps develop their own capability to keep customers informed on their trains, particularly given the lottery that long distance weekend travel has become.
(By "you" I don't mean you personally of course!!!)