A UK Fiancée Visa is a six (6) month visa issued by the British Embassy in Bangkok, which literally means that you have the intent to marry your Thai partner in the UK within the 6 month period of the visa. Once the marriage is completed in the UK, you will be granted further extension with no need to leave the UK. This extension is called FLR (Further Leave to Remain) which is awarded by the UK border agency and is currently valid for 2 years and 9 months, allowing your wife to be able to stay with you and employed legally in the UK.
Once you have registered your wedding in Thailand as well as the prenuptial then you can now apply for a UK spouse Visa for your Thai wife. Unlike a US visa for a Thai girlfriend the UK spouse visa allows you to take your wife back to the UK and will also allow her to work once all the paperwork has been completed. You can apply for the visa as soon as they have issued you with your marriage certificate.
You will note on this website the article on gay marriage or same-sex marriage in Thailand. Even though you cannot get married in Thailand as a same-sex couple you are allowed to live in the UK on a Civil Partner Visa. Same-sex couples who register a civil partnership are treat the same way legally as an opposite-sex relationship.