It’s clear that those other companies are permitted to decline ticket acceptance, specifically to prevent mass inflows of passengers they consider they cannot cope with.
In som circumstances, yes, they will be unable to accommodate all passengers the next train. But the majority of the time, they will be perfectly able to accommodate
some passengers on the next available train - and indeed they should and must so. Any 'excess' passengers can then be put on later services.
If push comes to shove, it will almost always be possible for the likes of LNER be able to arrange alternative transport or accommodation. The fact that these things might be expensive or inconvenient is not a basis for refusing assistance under condition 28.2.
This sounds somewhat fanciful. In practical terms how would this work in terms of physically buying the relevant tickets?
It's how GC et al used to operate in the past. They would have staff with ticket issuing equipment issuing tickets, or go round the TVMs and use the company card to pay for people's purchases. Whilst those are all perfectly feasible options, at least at the major stations, the bare minimum they should be doing is guaranteeing people that they will reimburse additional tickets that people have to buy.
Even beyond that, if TOC declines ticket acceptance that will be for good reason, and entirely at their own discretion. It would seem like sharp practice, not to mention dishonest in the extreme, for another company to deliberately circumvent the normal ticket acceptance process by purchasing hundreds or thousands of anytime tickets to effectively “dupe” the first TOC into accepting unacceptable levels of overcrowding.
It seems no more a sharp practice than the refusal of ticket acceptance in the first place. If LNER cancel one of their own trains, they can't limit passengers to a train in 3 hours time or whatever - people are entitled to get on the next train, regardless of how busy it is. Refusing ticket acceptance just because the train that's been cancelled was run by another operator is symbolic of the shambolic and disintegrated way the industry runs.
But that’s their decision, surely? Not to say they will always get it right, but are you suggesting TOCs are not free to make this decision?
Whilst in practice their decision on the ground is the one that will determine whether or not people are going to travel for free, it is always going to be amenable to legal challenge. If someone is (wrongly) told "we can't help you - our trains are too busy" and yet proceeds to buy a new ticket and is able to travel on LNER's train,
res ipsa loquitor...