What are the best ways to find a reliable immigration agent for a UK non-married partner visa for my Thai girlfriend?

You will have a Thai - England or the area London / Manchester / Newcastle etc web page where all the ladies that . You will get all the information you need off the ladies that have done or are doing the same . Good luck . Most important , read about it on the internet . Develope a story how and when you met , money you’ve deposited in her account . Photo’s of you meeting her family and vise versa . Lots of pic’s you visiting temples together . Put her in your will , dosent. Have to be 100% . 20 or 30 is fine Don’t deviate from your original story as they will have it all on file . Give them more info than they ask for . And you’re half way there . . Cheers and good luck . I did it for my lady to come to Oz , long drawn out and expensive process . But worth it in the end .

What visa are you talking about (33 month visa???)?

She has to do the A1 English test

You can’t just get a working visa for the U.K.

Must have a formal job offer from a licensed employer and a certificate of sponsorship.

Just buy a boat and sail from France

Agents don’t guarantee you will get the visa. They just ensure your paperwork is in order. Your gf needs good reason to return to Thailand - good job, regular income, savings, property in her name etc

The difficult part of this is not the process, it is going to come down to whether your girlfriend has good reason to return to Thailand. So does she have a good job in Thailand, a house, children, savings etc to demonstrate she will return. Without that, even the best agent will fail.