Correct, because it's done in your country and not issued by an embassy of the UK in Thailand. That's exactly what he asked for. Read: "Income Certificate from the embassy in Thailand (applicant’s nationality) - been told that this is not necessary? Is it and if so, why?"
You cannot get a certificate of earnings from your embassy because the United Kingdom is one of the countries whose embassies do not issue this certificate. Since you meet the requirements for a bank balance in Thailand, this is not necessary.
And the problem is that the visa is valid from the day it is issued and not from the day of entry. The applicant therefore does not want to lose too many days in the home country and submit the application as close to the flight as possible. To do this, everyone needs secure scheduling, which can only be obtained from the experience of each country. It would therefore make sense to create a list for each country or consulate where everyone enters the number of days from the application to the visa issuance.
Our "guru" is very humble, but I lack a word of gratitude for his detailed infos and thoughts on your stamp mess. Nobody expects you to worship him and kiss his feet.