I don't think we need to get into too much detail here, but the point is that each company has a choice to make regarding where and how tickets are checked; some companies (or the staff they employ!) make choices which will increase the likelihood of passengers either being unable to buy a ticket and/or be able to get away with not buying a ticket, or buying the wrong ticket. This is not necessarily linked to the operation of the train, nor which geographical area the train is operating in.
As for the suggestion to have separate gatelines at stations like Marylebone, I have split that into a new thread.
As for the suggestion to have separate gatelines at stations like Marylebone, I have split that into a new thread.