As far as I am aware, when your wife applies for a new passport in her married name, when you come to Thailand, she will be given her old passport back, which will contain her FLR stamp. With the return airline ticket there won't be a problem if she shows them her old and her new passports, and wedding certificate. The same will apply on reentry to UK, at immigration/passport control. She will need her old passport with FLR stamp in it, and her new one, and marriage certificate.
She will also need to change her name on her Thai ID card whilst in Thailand.
I think you would have to do this at her local amphur.
If any of this is mis-information, some of our better qualified members will correct me soon.