All the visas have changed since I lived and worked in Chiang Mai for 3 years. I had a non immigrant b and still needed to leave the country to get my work permit extended yearly and repot to immigration every 90 days. I was wanting to come back when I turn 60 and build up my Thai language skills and was looking at a student visa. Have you compared the student and Dtv visa? If so what are the advantages of the dtv visa?
If you only want to do a 1 month cooking class why don’t you just come on a tourist visa? I am not sure about British passport holders but for Australians we get a 60 day free tourist visa on arrival the are able to extend that for another 30 days.
I am sure you can then just enroll at a Thai cooking school for the months course.