JS wrote:

I suspect that because there are many National Rail stations where it
is not possible to set an OEP they cannot enforce penalty fares from
these stations.and as this would be even more confuriog, the TOCs can
only insist on OEPs on journeys starting from TfL stations.

Just a reminder - OEPs were insisted on by the TOCs as part of the
payg extension agreement. Penalty fares / prosecutions will be
issued / instigated by the TOCs and not TfL.

So the interesting legal point - can a TOC legally enforce a penalty fare if
it can be shown that the correct ticket was not available to passengers at
the starting station?