Not sure where the grey area is? They have Booked train only tickets with 2 Booked trains. They need to travel on their Booker trains.
It
is a grey area; where a ticket states "TOC & Connections", travel is allowed on appropriate connecting trains.
It is only in recent years that many connecting trains have become quasi-reserveable, in that you cannot actually reserve a seat, but they have "counted place" reservations, and therefore these "TOC & Connections" fares end up with an unwanted and unwarranted reservation for a relatively short distance connecting leg.
The counted place reservation system for local connections has had unintended consequences; there was clearly never any intention to require a passenger to wait at a station like Liverpool Street for a connecting service to finish their long distance journey just because the cross-London transfer times are incredibly generous.
In practise, I doubt any Abellio Greater Anglia Guard or RPI is going to actually mind a customer using an "appropriate" connecting service that is not the one the customer has been unwittingly issued a reservation for; most staff in such a position are going to take a
pragmatic view and see the bigger picture.
I have never been refused travel on a train that is clearly an appropriate service by any reasonable definition of the word.
Furthermore, I know it is the policy of TPE to allow travel on appropriate services other than the reserved service to connect with longer distance operators for longer journeys e.g. Garforth to London King's Cross etc.
Some jobs require pragmatism; some don't. Some people are suited to such jobs and some aren't. In my experience
most Train Managers
are pragmatic.
Which is all well and good until your train into London is late and you miss your connection and have to buy new walk up tickets..
Presumably you are referring to the specific scenario of a passenger booking a combination of Advance tickets for trains that, together, do not form a valid itinerary due to the timings not meeting the minimum interchange time.