As a British citizen you can enter Thailand for 14 days without visa.
Bermuda is not a country but a “British Overseas Territory”. Visa exemption is (a.o.) for holders of passports of Great Britain and its overseas territories.
I am confused. I was told just a few weeks ago that you can apply for a 90 day non-O visa within Thailand, but now you seem to write that after entering on a tourist visa one must leave Thailand and apply at an embassy outside of Thailand...
What is the real procedure and where can I find positive confirmation on official sites?
You get that support letter only when you can show receipt of 3 months retirement money. I am moving almost immediately after my retirement. Besides, I won’t count on getting regular income from a government or an insurance organisation. The Dutch government is as reliable as the Dutch weather (i.e. not reliable at all) and the EU is eating all Dutch pension plans, so I am not going to rely on an income statement. Money in the bank is the only thing that counts.