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Euston LU refused it, and wouldn't even accept it when told to do so by phone from the ticket office upstairs.
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Devil's advocate here:
Why should they?
Just as National Rail TOC staff have very little LU ticket training, London Underground staff are shown that either a ticket must say Zone Ux, have a Maltese Cross, etc -- or during disruption to accept tickets on behalf of X TOC. If National Rail as a whole can't get it right, that is their fault, not LU!
For example if LU machines did not print the correct zones on a Travelcard* - I would not be accepting it unless specifically briefed by my company, to put things the other way round.
*For example, Gold Record Cards for Zones 1-9 + Watford Junction just print "123456789" - luckily I can scan the Oyster card to find the correct zones!