CJG writes
There are plenty of ways to 'cross' London without going through zone
1 on the tube: buses, N London line, S London trains.


But if you just have a single ticket it might look a bit suspicious.
Just out of interest. If you were to buy a ticket from Zone 5 to Zone 4
not via Zone 1.
And then travel from a Zone 5 station in West London. To say a Zone 4
station in East London. But going through Zone 1. Would the automatic
gate pick up on it? Or just read you brought a ticket to Zone 4 and let
you through?

The ticket machine would know from which station the ticket was
purchased and act accordingly. For humans, different coloured triangles
help to highlight this scenario.

--
Dave