I put the scenario we faced to
GWR▸ via social media, asking how I can pay. Their response:
Hi, if you can't purchase a ticket at the origin station you can buy on board or at your destination. As you have already made the journey we can't force you to make a payment retrospectively, but we would ask you make a charitable donation to our partner charity Prostate Cancer UK▸ .
So, later this week that's exactly what I shall do. A decent common sense response there from GWR. That gets my praise.