If you study the public timetable very carefully, you will find first thing on Sundays mornings at start of service there are a couple of booked workings into St.Pancras "high level". These have always been in the timetable. Same when 319s ruled, there were always early Sundays mornings into STP high level. Of course, as any Sunday morning train, these can sometimes be altered by engineering works, can be buses, or as trains diverted into the low level on some dates, but nonetheless high level is booked. IIMU it is exactly for the reason of retaining route knowledge of Bedford etc TL north depots irrespective of whether the tunnel route is blocked or not.
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Nick