Alan Osborn wrote:

How far out of London can you buy a combined London return fare inclusive of
a Travelcard. When I travel to London from Oxford I always ask for a London
Travelcard, that gives me a return fare to London plus an all zone
travelcard.

I'm not sure for a day Travelcard, but I believe GNER used to sell
monthlies from Berwick-upon-Tweed. They probably still do but I don't
feel like checking the NFM right now.
--
Michael Hoffman