A UK fiancee visa allows you to bring their Thai Fiance to join you in the United Kingdom as long as they themselves have settled status as a permanent resident or British citizen.
Settled status here means that they have been living here lawfully with no time limit on their stay. If you are returning to the United Kingdom to settle, your partner can also apply for a UK Fiancee visa to join you at the same time.
If you wish to bring your Thai wife see the section on UK Marriage Visa.
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