BNO passports are different from standard UK ones - they are basically issued to people from Hong Kong born before 1997. You can go there for 15 days visa free with a standard British passport.
My immigration office is Samut Prakan. I actually hadn't left Thailand in over 2 years since before COVID, but previous to that, whenever I left Thailand and returned, I had to go to Samut Prakan immigration within 24 hours of my arrival in Thailand and get a stamp with the date and immigration officers signature on the receipt of notification ( from TM30 form). I was just wondering if we still had to do that as I heard some rules were relaxed but not sure which ones exactly. The new immigration office is a real pain in the ass to get to.
What I'm saying is that foreigners are excluded from the new laws, but for someone like you who needs it for medical treatment you'd need to obtain a licence from the FDA. Cultivating for personal use in my book means possess. But you can ask about it when you get your licence. IF not, no big deal you can just buy from a shop.
Visitors coming for ganja therapy must first obtain a license from the FDA to “import, export and possess” the necessary amounts of marijuana prescribed for treatment.
A quick google search answered your questions. You can cultivate it at home but you'd need to register first. Also you must only use it for home use and not smoke in public as it comes with a jail sentence if they catch you.
I live in Bangkok, you could find some places that would cater to you, but you'd be best off living in a place like Pattaya or Phuket where all the expat stores are. Also central Bangkok is good for expats, but outside the central hub, you'd need good Thai language skills. Phuket and Pattaya you won't need to worry about that.