"G" wrote

I can support that with my experience on the Piccadilly line with a
replacement bus between H&I and Seven Sisters - the tube barriers were
open to exit and enter the stations and no one touched in on the
buses.

I'm not quite sure when the Piccadilly Line served Seven Sisters. Leaving
the barriers open is fine for passengers who transfer from tube to
replacement bus then back to tube, but what about passengers who end their
journey at an intermediate station served by the bus? Unless it is made
clear that they must touch out when they leave the tube (although other
passengers are encouraged not to) they will get an unresolved journey.

Peter