It’s very simple! Appeal the PF on the abundantly simple basis that a valid travel ticket was shown, and this protects the OP from prosecution and is a ground certain to succeed anyway.
I don't think that is quite right - at least not until three weeks have passed from the day on which the appeal is received.
You have to read the whole of Regulation 11 (as amended by the 2022 Regulations). It states the following:
11 Penalty fares and criminal offences
(1) A person who has been charged a penalty fare under regulation 5(1) is not liable to pay where proceedings for any of the offences specified in paragraph (4) are also brought against that person in relation to the same failure to produce a platform ticket or a valid travel ticket which gave rise to that penalty fare.
(2) [Subject to paragraph (2A),] If a person to whom paragraph (1) applies has paid the penalty fare, or part of it, the operator on whose behalf the penalty fare was charged must refund that person the amount paid within the period of 10 working days, beginning with the day on which proceedings are brought.
[(2A) If a person to whom paragraph (1) applies has paid a penalty fare calculated in accordance with regulation 9(1A) or (4A), or part of such penalty fare, the operator on whose behalf the penalty fare was charged must refund the person any amount paid that exceeds the amount of the full single fare applicable for their journey within the period of 10 working days, beginning with the day on which proceedings are brought.]
(3) Where a person falling within paragraph (1)(a) has appealed against the penalty fare under regulation 16, proceedings for any of the offences specified in paragraph (4) may only be brought against that person for the same failure to produce a platform ticket or a valid travel ticket if the operator, on whose behalf the penalty fare was charged, notifies the relevant Appeal Panel that the penalty fare is cancelled before—
(a) the relevant Appeal Panel has decided the outcome of the appeal under regulation 16; or
(b) the time period mentioned in paragraph 6 of Schedule 2 expires,
whichever is sooner.
So under 11 (3) if the person appeals then the operator can still bring a prosecution if either the outcome of the appeal has not been decided or before the the time period mentioned in paragraph 6 of Schedule 2 expires, which is 21 days beginning with the day on which the appeal is received.
Ironically you are better off appealing then paying. If you appeal after that 21 days you can't be prsoecuted. If you pay then under 11 (2) the operator simply has to refund the amount within the period of 10 working days, beginning with the day on which proceedings are brought.
That isn't my interpretation of this clause.
(3) Where a person falling within paragraph (1)(a) has appealed against the penalty fare under regulation 16, proceedings for any of the offences specified in paragraph (4) may only be brought against that person for the same failure to produce a platform ticket or a valid travel ticket if the operator, on whose behalf the penalty fare was charged, notifies the relevant Appeal Panel that the penalty fare is cancelled before—
(a)the relevant Appeal Panel has decided the outcome of the appeal under regulation 16; or
(b)the time period mentioned in paragraph 6 of Schedule 2 expires,
whichever is sooner.
Agreed - it is either once the appeal has been decided or before the the time period mentioned in paragraph 6 of Schedule 2 expires, which is 21 days beginning with the day on which the appeal is received.