Travelcard then no issue. If some PAYG is required to cover the part not
covered by your Travelcard zones then the gate should calculate just a
LU gate does. If you are exiting and your Travelcard does not cover
Clapham Junction and you can't reasonably have got there on Overground
given your entry point then I'd imagine your card would be rejected.
Might need some more thought on this latter point given that many entry
points have no gates or validators - hmm.

It will become *much* more fun when PAYG extends to other lines and the
route and zone combinations become more complex.
--
Paul C

Given the interchanges possible at West Brompton etc, the only
journeys that could be rejected are ones to and from Wimbledon or
Richmond that are too quick.