Yes, it depends on the TOC. EMT staff at St Pancras are clear that the policy is not to let people board an earlier train if their booked train is cancelled (unless there is mass disruption and all restrictions are lifted). EMT obviously prefer to have disgruntled passengers hanging around and then paying out Delay Repay afterwards rather than letting happy people travel on the earlier train and having no liability to pay compensation.
The suggestion
The suggestion
is probably true. The problem with that at St Pancras is that the gate staff prevent passengers getting to the guard to ask.However most guards would use common sense and use discretion