Welcome the the forum Oxonhutch!
It is indeed an
imperfect system as your was family member was told.
They were given permission by staff at Reading to travel on an earlier service, so this should carry some weight if any dispute arose later in the journey. In these cases it is always worth having the ticket(s) endorsed, if possible with the station stamp. There is room on the rear of tickets for such endorsements.
There's little, if anything, in the public domain that passengers can refer to for absolutely clarity in the scenario you've highlighted. There is however internal staff guidance on what they should allow passengers to do when they are travelling with Advance Purchase tickets and are facing delays. That guidance to staff is called the '
Advance Fares FAQs▸ ' a .pdf of which is attached to this post.
From that you can see Q22:
Q22. Can a passenger travel on any other service than the one on which they are reserved, without changing the booking?
A. the following principles apply.
1) Start of the Journey. It is the passenger responsibility to turn up at the start of the journey in time for the departure of the first train. If they miss it due to parking problems, taxis not turning up, etc, they must buy a new ticket,
2) Once the journey has begun. If the passenger is delayed and the train company or it's partners [are] at fault, which should be check by [staff] control office, change to a train of the same company is allowed to get them to their destination with the least delay. This is irrespective of the combination of rail tickets held. Examples are:
Included: are passsengers on valid:
Through domestic or international tickets. e.g. Brighton-Scarborough route TOC▸ X & connections;
Through rail and partner tickets for which there is a through bus, tube, ferry or metro fare, e.g. Zone U12-Leeds, Wisbeach coach-York, Ryde Pier-Hull, etc;
Combination of rail only tickets.
e.g. Rail season ticket Skipton-Leeds and Advance Leeds-Peterborough, or adjoining advance fares;
Combination of rail and partner tickets.
e.g. Brighton-Zone U12 plus advance London-Manchester, or;
e.g. Advance ticket Bristol-Paddington plus tube single ticket, plus advance ticket Kings Cross to Hull;
All Zones Travelcards, PTE▸ -products (where rail is included) plus advance fares, etc;
Combination of Eurostar tickets into the UK▸ and then either advance tickets from London terminals or "London Intnl CIV▸ " or Lndon Eurostar CIV;
Not included for the avoidance of doubt, are:
Non train company travel on separate tickets, e.g. tickets that begin on bus-only, tube-only, ferry-only or metro-only tickets. (this includes "PlusBus", which is a local day-rover bus ticket not compatible with a medium/long distance advance single ticket, so are kept as separate tickets), or;
Tickets that cannot be read on-train e.g. smartcards (allowable where electronically checked, verified and advance ticket endorsed in travel centres).
Please note that there is no change to :
a) Any other rules e.g. trains stopping where tickets join together (NRCoC▸ 19), nor;
b) The need to verify that a train has been delayed on route and ticket endorsed, nor;
c) Any other passenger entitlements as defined by the NRCoC or CIV conditions, nor;
d) General ATOC» disruption guidance
e) [Staff] discretion in extreme circumstances.
Then there is Q25:
Q25. Can passengers on an advance ticket travel on earlier connecting trains?
A. Yes if it is non-reservable, no if it is reservable.
An exception that benefits customers: Where East Coast is a connecting TOC from Stevenage via London and vice versa e.g "AP London Reading" or "VWC & connections", East Coast waives the need for travel on the exact East Coast train booked on this relatively short journey Stevenage-Kings Cross, even though retail systems will force a reservation to be made.
There you can see an example of the shorter leg not requiring travel on the exact booked train. Perhaps in the case you highlighted First Great Western also have internal guidance given to staff to show discretion and allow travel on an earlier service, whether it is reservable or not. Advance Purchase fares are not offered by
FGW▸ for journeys solely from Reading to London, but seat reservations will be given by the
imperfect system for Advance Purchase fares that include Reading to London as part of a longer journey.