I am not referring to the OP when I say this, but one can quite understand why somebody on the autistic spectrum or who suffers from social anxiety may wish to avoid debates around validity in order to exit a station.
but how do you expect to manage in life if you cant deal with someone asking why your ticket doesn't work the barrier? You have to develop coping strategies and train yourself to use them. Not easy but essential.
Some people assume that something must be up because it doesn't work the barrier.
of course they do. That is their job. Avoid the situation by approaching a person on the gateline and explaining politely and calmly that the ticket doesn't work the barrier. You don't need a long discussion about ticketing rules minutiae. Just explain why it doesn't work. It happen to me often if I buy on the Marston Vale line.
Don't allow something simple to derail your day out. Just keep calm and relaxed.
And to get back to the central point: The Crewe - Euston London Mildand services tend to come into an ungated platform at Euston. Tend to. Not certain to.