Applying for UK visa from Thailand can be problematic. If you are getting married in Thailand to your fiancee then you need to register your marriage before you can apply for a UK spouse visa. If you are only engaged to get married then you can still apply for a UK fiancee visa. There is also a visitor visa as well as a UK partner visa for the UK if you wish to tour the UK or move back to the UK with your partner. Also ensure that the UK spouse visa requirements have been met when making an application.
Note that there are 3 popular UK Visas which are applied for in Thailand. The visitors visa is the most popular and is also the most rejected visa as paperwork may be incomplete or the person applying many have been banned from the UK for overstaying their prior visa.
UK Visitor Visa
This is a short term visa for tourism or to visit family who live in the UK. They are going to want you to show that you will not overstay your visa. After the first UK visa the next one tends to be easier as you had been to the UK before. You will need to show employment, property and anything else that will want to make you return to Thailand.
Marriage and Fiancee Visa
The UK fiancee visa is the most common visa for Britain after the tourist visa from Thailand. You will need to show that you have accommodation as well as an income you get your Thai fiancee her visa. Once there you will need to get married and have her visa status changed. This does take a while however the application process requires a criminal record check as well as a medical checkup. Most of these do get approved. Those application where you get a visa denial are rare but they do occur.
UK Marriage Visa
Not as common as the fiancee visa for the UK. You will need to have all the correct documentation. Most tend to return to the UK first if they had retired in Thailand. They return to the UK yo obtain accommodation and employment. Then apply to bring their Thai wife back to the UK. It is very rare that the person arrived as a tourist and then got married in Thailand. Most applications tend to be those who have lived in Thailand for an extended period of time either retired or working.
See also: How to apply for a UK visa from Thailand as well as the US Visa for children and everything Pattaya you would need to know. There is also the Australian fiancee visa for a Thai as we have it here.
Firstly if you had been considering visiting the UK as a Thai national then this is the best place to start with the visa application. If you are engaged to a Thai national then you can take her back to the UK with a British Fiancee Visa is needed. If you have applied for a UK Spouse visa then these are the common problems in Thailand and also the process. Finally when you apply for UK visa from Bangkok ensure you have proper legal assistance.