My suggestion would be to contact Scope the disability charity (disclaimer I work for the charity). Contact centre can provide advice by phone or email https://www.scope.org.uk/advice-and-support/ or there is an online community where you may have find some insight and suggestions - https://community.scope.org.uk/Thanks Richard - I've registered on the forum and asked the question on there. It does seem to be a lively group, so maybe there will be some advice there - if not I'll contact Scope directly.
They also suggested that even if assistance isn't booked (impulse travel) throwing oneself on the mercy of the assistance desk (at a major station) or Random Staff Member (at a smaller station) generally works.But how well does that work at unstaffed stations? How do train staff tend to react when unexpectedly finding a wheelchair user waiting to board? Maybe one to ask these Scope people.