As I recall, the train from Patchway to Reading was indeed a HST diverted via Bristol Temple Meads, and not stopping at Bristol Parkway at all, owing to engineering works between Bristol Parkway and Swindon.
Having previously lived in Patchway, I once travelled on such a direct Patchway (via Bristol Temple Meads) to Reading HST (with the train then going on to Paddington) service myself.
Perhaps, on reflection, the use of the word "ransom" in my previous post was a bit strong.
However, it does seem that there is something of a 'Jekyll and Hyde' divide between customer service departments who seem to think that "the customer is always right" (I'm sure Flamingo can supply examples

) and certain prosecution departments who seem to think that "the passenger is always lying, thieving, scum", even when, in individual cases, there is considerable evidence to the contrary.
In some cases, a passenger who has, or may conceivably have, a viable defence (as, arguably, did the CAG poster, if my memory is correct) may nonetheless feel pressured into paying the PF/ 'out of court settlement' because they do not want the time, stress and hassle of dealing with the likely prosecution if it is not settled, nor to risk a possible large fine and criminal record if the magistrates decided to agree with bb21's interpretation and so to convict under s.5(3) Regulation of Railways Act 1889.
In my view, this is highly unfortunate, although, on reflection, I'm not sure what exactly can realistically be done about this; it is not really different from a (hypothetical) person falsely accused of mobile phone use by the police paying the £100 FPN (with three points) regardless because they do not fancy their chances against the word of a police officer in a magistrates' court.
I would support a rise in the PF to £80, in return for certain existing 'grey areas' being resolved in favour of the passenger (e.g. a cash-only passenger boarding at a station with merely a card-only TVM), and for a guarantee that no passenger be left 'out of pocket', relative to the fare they ought to have paid, as a result of being incorrectly issued a PF.
For example, if a passenger genuinely boarding at a station with no purchase facilities, who should have been sold a CDR, is instead charged a PF for the outward journey, and then has to buy a CDS for their return journey at 10p less than a CDR, then merely refunding (or cancelling) the difference between the PF and a SDS would seem to be inadequate restitution for the passenger. This may be even more obvious if the passenger is then made to buy a full priced SDS from the next stop in addition to being charged a PF.