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Established Member
For other ticket types you should be able to 'check in' and book your seat almost up to departure from the originating station. This is not perfect but would cut down on 'no shows'.
Again very hard to do on Aberdeen / Inverness - London services. Someone at York may need to get up very early to check in, and if the preceeding service is running late may decide to board it anyway.
Some operators wanted to sell tickets after the train had departed to fill up an empty seat on route, but this was rejected as it may result in the person with a walk on ticket that had been on from the departure point getting kicked out of thier seat. Imagine a Edinburgh - Penzance journey and getting kicked out a Bristol!