There’s a few got visas based on multiple appointments from a Thai dental clinic, these MUST be actual appointments and follow up treatments with dates from the clinic. As for the UK dentist but you haven’t got a chance.
As Thailand has signed up to CRS they have access to your other countries tax returns and bank accounts. You pay the difference between what is already paid and what becomes due in Thailand due to the different tax bands and percentages. It isn’t a new law in Thailand just the full interpretation of the existing ones so nothing needs to be approved by the Thai government it’s up to the revenue department to just go for it.