I was in Thailand 🇹🇠when I got mine from the British Consulate there. It was such a long time ago but I recall it took about a month to get that affirmation. I am not sure if you'd be able to get the affirmation from here before you go?
I recently went through the process and can advise. I converted my civil partnership to marriage a few years back.
First while in the UK you need to get your marriage cert apostilled and attested by the Thai embassy. Google is your friend here as there are many to choose from. At this point it might be a good idea to get the marriage cert translated too or you'll end up having to do that in Thailand which is what I did . I used Sawadee Translations who translated the cert and then met me at the Consulate of Foreign Affairs at Chaeng Wattana to submit the docs which were then ready to collect 3 days later. You could do this part yourself to save money.
please do it yourself. Pop in to your local immigration office for a list of the requirements needed for your visa. Each office has its own requirements, so best get their list. Once you've done it, you will never think of using an agent again.
Having recently been through this process this is what I did. We were married in the UK. I had our UK marriage cert apostilled and attested by the Thai embassy in the UK. In Thailand I then had the attested marriage cert translated and stamped by the Consular of foreign affairs at Chaeng Wattana in Bangkok. You have to take the translated cert and original to the Consulate and they will ask you to come back 3 days later to collect said document. You can then take these docs to your local Amphur and get them to list the marriage. This is usually done within an hour or so.
In that case, I think each Immigration office makes up its own rules. I was based in Sriracha at the time and back then there were only a handful of foreigners in the area.