Is a Royal Mail first class signed-for envelope sufficient for returning my passport from the Thai embassy?

The user inquires about the sufficiency of a Royal Mail first class signed-for envelope for the return of their passport after applying for a Thai tourist visa. The conversation reveals that the user has already sent off their passport to the Thai embassy in London and included a self-addressed envelope. Other commenters express confidence that this method is acceptable, while also discussing additional fee considerations for the visa application process.