The difference between a UK fiancee visa application and a UK settlement visa application comes down to one thing mainly whether you plan to marry your Thai partner in Thailand or in the UK. If you plan to marry in the UK to your Thai partner then you would apply for a 6 month UK Fiancee visa and then get an extension after marriage in the UK and a UK Settlement visa means you are married in Thailand already check links http:///   Settlement visa to the UK  http:///