Pre-requisite to Answering: Chiang Mai (British Passport)
Has anyone used the embassy issued personalised Covid-18 Extension letter at Chiang Mai immigration?
(I was issued a personalised version of the letter today by the UK Embassy Bangkok)
I asked about having to have an original copy, and the UK embassy assured me a copy is suffice (with personal details listed) & for immigration to call them if they don't accept.
(Sorry if this has already been answered in a comment.)
Thanks,
TLDR : Answer Summary
Yes, you can use the personalised Covid-19 extension letter issued by the UK Embassy to obtain an extension in Chiang Mai. Make sure to print the letter in color along with the email confirmation from the embassy. Note that as of the mentioned date, all immigration business will be conducted at the main immigration office near the airport.