Which used to be signed, but lost it, when you have ascended the first east end escalator you go left into a passage, but left again to a now unmarked lift in the corner. Go up in that and it comes out in a further passage going to the very west end of the Westbound Inner Rail platform.I feel honoured to be called out like this
While not quite what you asked in the OP, just to mention as per our initial discussion that in a similar vein if you want to get from the Elizabeth Line (or Thameslink but I'm not sure whether it's quicker to go via surface or not) to Barbican there's no need to change onto the sub surface lines - there's an exit from the Farringdon Lizzy Line platforms directly into Barbican station itself.
I wonder what the longest overall passage is. I suspect Liverpool Street station concourse entrance, down to the Liz line, full platform length, up to Moorgate passages, onto the Northern Line, must be the best part of a mile.