Presumably some level of "off premises" work does go on to catch people who donut hole or deliberately buy short tickets, to verify that they are continuing to do it.
And presumably a defence against donut holing is to produce tickets for the whole journey, however that could be many days/weeks of tickets to produce.
so it seems in that case, you have no choice but to retain any rail journey ticket until the statue of limitations runs out.
Looking at this with common sense and in a logical manner, a person leaving a station will have their ticket with them unless it has been collected (in which case there would be NO RPI), or they are inconsiderate litterers who choose to discard their ticket during the journey or on the floor at the station.
Any challenge would happen in the immediate vicinity of the station entrance, rather than down the High St or the shopping centre. On that basis once a person has left Railway premises and not been challenged then they are free to discard their ticket.
I can tell you now that asking someone to produce a valid ticket in the local Tesco or Sainsbury's is not a scenario contemplated other than in the mind of the complainant. The Magistrates would consider that the journey had long ended at that stage, and are unlikely to deal with the situation unless the person had run away and was apprehended away from the station. Remember that as fare evasion is a Criminal Offence, pursuit is permitted in such cases.
Knowing Southall as well as I do, I can anticipate that there were a number of people passing out through the entrance, and the challenge was made in the immediate vicinity of the station, which given the possible scenario would be seen as being reasonable.
It really is not the case that RPIs hang around in shop doorways further along the street waiting to trap unwary passengers. I anticipate that there was a group of RPIs outside the doors, and to be frank with you the whole subject is actually a non-issue turned intom a drama because the person who complained in the first place was in a bad humour, and took exception to being asked for their ticket as they left.