Presumably such tight to gauge tunnels exist on the section of line
between Kentish Town and Farringdon.

It's a double-track tunnel.

I wonder if the decision is based upon the premise that if two trains,
travelling in opposite directions, became stuck in the tunnels it would be
impossible to detrain passengers using the side doors, either to the cess
or
the six foot. Hence the requirement for end doors.

I was wondering if perhaps the widened lines from Moorgate to King's Cross
Thameslink were still considered as part of the London Underground system at
the time and so found themselves having to meet extra requirements because
of that, regardless of the actual practicalities.

Do we actually know if the same restriction applies to this day? The 319s
have been around quite a while after all.