I'm going to get a 1 entry tourist visa from the Hull consulate (UK).

I know that this will give me 60 days but I think I may need 70-80 days, is it possible to go to Mae Sai and get another 30 days with a visa exempt stamp since the new changes rather than getting a double entry and still having to cross a border?

Yes, you can get a 30-day extension of stay, and this is indeed the most practical way for your plan of an uninterrupted stay of 80 days in Thailand.

You could get a 30 days extension of stay within Thailand at an immigration office near your place of stay, no need for a border run for that..

The cost of the 30 day extension is 1900 baht. You will have to fill in a TM 7 form with one picture.

Ah, yes, I forgot to mention that the 30-day extension I was talking about is to be obtained at the local immigration office. No need to make a border run for a visa-exempt entry. (I hadn't read the OP carefully enough) Thank you, KhunKone, for mentioning it.

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