Sorry you received such a response from National Rail.
I'm baffled by their reply, considering I have a Disabled Persons Railcard for my Autism, as they stated it was eligible, alongside my proof of entitlement. For that, I submitted a letter of my diagnosis from the 1990's, and my PIP letter.
I also remember boarding a faster train that my off-peak ticket wasn't eligible for. Not realising this when I showed the ticket inspector both the ticket and my Disabled Persons Railcard, he saw how anxious I grew when he was initially vexed at what I'd done wrong, and proceeded to take me to a quieter area of the carriage to explain to me in a friendly non-condescending way.
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"I may not have gone where I intended to go, but I think I have ended up where I intended to be."