The marriage will only be recognised under UK law if it is valid under Thai law. For it to be valid in Thai law, the marriage must be registered with the Registrar at an Amphur Office (District Office). A list of some Amphur offices in Bangkok and upcountry is attached.
- Attend the British embassy in Bangkok
- Submit an affirmation of freedom to marry
- Get the affirmation translate into Thai
- Visit the ministry of foreign affairs
- The document is stampe & authenticated
- Enter any amphur, then marry
- You will both be issue with Thai marriage certificates
Do all of the above then the marriage then the marriage is legal in the UK.