I wondered at the time what would have happened if I had travelled in first class, and asked to be sold a first class ticket, given that:
1) Patchway has no purchase facilities whatsoever (so PFs are out of the question);
2) There is no first class fare between Patchway and Bristol Temple Meads, and;
3) The old 1.5 multiplier has been officially abolished
I have seen a similar situation. The guard explained that there was no first class fare available for their journey, so the said that they would sell them a standard class ticket and they could have a free upgrade to first on this occasion only. I would not expect that treatment generally though!