there was a question mark there for a reason. Last time I came in the immigration officer said they're now cracking down on anyone staying more than 180 days a year
Not from the UK but fellow European in Thailand. You can just get yourself an eSIM from AIS here. I would advise you to keep your UK sim as a physical one. European eSIMs are almost impossible to activate outside of Europe, physical SIMs are easier. If ever you phone breaks down you just swap the SIM and you're back in the air. I would get a prepaid card for the UK as you're not likely to use it for much else than receive SMS