"Clive Coleman." wrote

But in what scenario might this option be any better than a road or air
evacuation, or staying out? I can't imagine one.

Gas or other airborne attack.

But in any sort of mass attack, the Tubes are going to be paralysed by (a)
general panic; (b) absenteeism of key workers; (c) breakdown of supporting
infrastructure (communications, signalling, electricity) consequent on the
foregoing.

None of that applies to helo or road evacuation. And I bet there is a
gas-tight limo somewhere....