"Andrew Clark" wrote

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....

In any sort of mass attack the road network would be paralysed by general
gridlock, which police outriders would be unlikely to be able to carve a way
through for a VIP (general gridlock happened following 9/11 and 7/7). So
helos it is.

Peter