You can request it online and send your passport to the British Embassy. I did mine that way a few months ago and it came back in about a week. I don't know where you live, but I was lucky as I live in Hua Hin and could access the Ministry of Foreign Affairs here.
so are you saying on the retirement extension, your balance can't go below 800,000 to 400,000 three months after the extension and return to 800,000 two months before the next extension?
this is not recommended as even agencies are struggling to open bank accounts this year on a tourist visa, so the price has increased. Much easier to get the non-O before coming to Thailand.