If you are getting married in Thailand as a British national, then you will have to obtain a Letter of Affirmation or also known as the Affirmation of Freedom to Marry from the local British Embassy. In order to start the process you will need to contact the British Embassy in Bangkok to ensure that they are open and how long this would take to complete.
Note that the Embassy is closed on British and Thai national holidays. What you need is mainly your passport if you have never been married. If you have been married before then you will need to take your Divorce Decree/ Death Certificate (if applicable) with you. They need to see the original copy of your documents. If you do not have these then you will need to find a way of obtaining them.
Once you have completed the needed form and provided all your documents such as your passport, divorce decree or death certificate, they will normally issue the Freedom to Marry the following day. Then, you will need to take this document with you to have it translated and certified. You can ask a lawyer in Thailand to do this for you. Once done, the documents including the death certificate or divorce decree needs to be taken to the Thai Foreign Affairs Department outside of Bangkok to have the documents verified and signed. This takes a few hours to complete and they will place a stamp on the documents to show that they are authentic and have been verified by the department.
If you have a prenuptial agreement then now is the time to provide this as well, as it should be provided and registered at the time of the marriage registration. Note, that you cannot file a prenuptial agreement once the marriage has been registered. It has to be handed in at the same time as the local Amphur’s Office.
All your documents which have been verified by the Thai Foreign Office now need to be taken to the Amphur’s Office or local District Office to have the certificates issued. There are two marriage certificates issued in Thailand. There is a diploma style certificate, as well as a multipage document. Your marriage is now registered.
If you wish to convert your Thai visa to a marriage visa, this can now be done with the certificate on hand. You can also ask a lawyer in Thailand to assist you in obtaining a UK spouse visa now that you are married and wanting to take your Thai wife back to the UK.
Most expats who do get married in Thailand tend to stay for long term. The marriage visa has an easier process than the retirement visa, if you are not yet over the age of 50. Take proper legal advice in Thailand when it comes to marriage as property in Thailand also becomes an issue. Marriage laws in Thailand as well as Divorce laws in Thailand are not the same as back in the UK.