In message , Tim Roll-Pickering
writes

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?

Probably not (or at least the PF can be appealed against). The DfT make
the situation clear he

http://www.dft.gov.uk/pgr/rail/legis...policya?page=7

But with OEPs rapidly becoming available on automatic ticket machines
across London, it would probably be necessary to establish that the
ticket machine was broken and there was no other one available nearby.

Of course, if the ticket office staff are asked for an OEP and they
claim never to have heard of such a thing - even to the extent of saying
"you'll have to use the machine over there", I think one might enjoy a
very successful appeal.
--
Paul Terry