I didnt see reference to non O visa in OP only that they move to thailand on 1st November. I only attempted to clarify your statement that they would get 90 days of stay on entry to Thailand,
Single entry visa on entry gets you 60 days of stay but can be extended at Thai immigration for a further 30 day at cost of 1900bt thus giving you 90 days stay
But if thai immigration did'nt give the extension then they would give 7 days get out of thailand stamp, so I think the OP will be OK in this scenario 🤔😁
No issues boarding the flight, I have done this and explained to UK check in staff that I will be extending my Visa exempt after I have been in the Kingdom 2 weeks.