Go to your embassy, get it certified, then you have it translated, finally go to Thai Consular Services to have it legalized. Then visit you’re amphur where you’re living to get it registered.
Immigration aren’t interested in what you used the money for just that it dropped below the minimum required 400k. In allowing that to happen you have failed to continue to meet the requirements for which the extension was granted. You’ll need to apply for a new visa and start over.
Happy Born Day Tod. Have a great day. Your willingness to share your extensive knowledge has helped countless people over many years. Keep up the great work👍
For what it's worth this is how it works for me from NZ (I assume it will be similar for UK). Get the marriage certificate authenticated in the UK. When you arrive in Thailand it needs to be certified, translated and legalized (in that order). For me the NZ embassy certify, you then take it to a translation service and then to Foreign Affairs for legalizing. You then take it to your local Amphur where you'll be living to get the kor ror 22. Contact your embassy in Bangkok, I'm sure they'll put you on the right track. Hope this helps.