If you are getting married in Thailand (See also here) then you will need to complete the following process for the British Embassy in Bangkok in order to get married. This is from the British FCO in the United Kingdom. There is a PDF made for this process while in Thailand.
These are the steps:
- Check the application fees for the letter of affirmation on the gov.uk website. (Currently £50)
- Make an appointment at the Embassy in Thailand;
- Complete the letter of affirmation, bring with your UK passport and fees;
- Bring with original divorce decree and death certificate where applicable;
- All documents must be certified and translated if in another language other than English;
- The letter of affirmation must then be legalised at the Thai Foreign Affairs Department;
- Documents must then be taken to the local amphur’s office for marriage registration;
That is the marriage registration process in Thailand. Speak to a Thai lawyer for more information if you are not certain. Once you have registered your marriage you can now apply for a UK marriage visa or UK fiancee visa. Note the fees for the visa as well as taking any of your Thai wifes children back to the UK would require a dependency visa as well. Search this website for more details. You Thai wife will also need a TB test in Thailand.