There is no real answer to this question but my advice is it is always better to go to the UK on holiday first before applying for something more permanent and the reason is it is better if your Thai partner experiences the differences in the weather, culture, food and has the chance to meet your family in the UK and mainly it will show the Entry Clearance Officer at the British Embassy that when your Thai partner returns after their holiday they have come back to Thailand and also they must be serious about the future with you. Getting your foot on the first rung of the Immigration ladder is important.