The law is quite clear. If a genuine error (and not a case of trying to lure you in with a low price) then the retailer can charge the correct price, the customers "right" is to either pay the correct price or not complete the purchase and there being no penalty. However, good firms will honour the incorrect price for the purposes of good customer services. That said, a few years ago Tesco inadvertently left two zeros off the end of the price on the new ipad and made everyone who had preordered at the incorrect price either pay the correct price or be refunded their deposit. This was, of course, before the products were collected/delivered.
I think the railway would be on dubious ground to say a ticket it had sold and given to its customer was no longer valid. Possibly between payment and collection, but after collection it wound be very difficult to cancel it. I'm glad to see the GWR lawyers clearly agree and have said to accept them. Albeit they are no longer available.