Daniel wrote:

Could there also be some cross-london journeys where travelcard
holders without the central zones on their tickets could require two
(or more) OEPs (due to the original OEP being cancelled by a gateline
en-route)? I suspect that ticket-holders with this type of ticket
could see some non-optimal charges as well.

Not necessarily because an OEP appears to be a one way requirement.
If you initially touch in 'outside' your TC zones clearly you must be
travelling on PAYG already when you reach your zones.

But what happens if you pass through your zones and out the other side must
depend on if you have to cross a barrier line en route as you suggest. Not
sure on that one...

Paul S