I'm reluctant to post the original marriage and birth certificates because I will need them for use in Thailand around the same time. Can someone please tell me where and who can provide official certified copies (NOT translations!) that the UK government will accept? I'm hoping can be done in Sakhon Nakhon, rather than have to trek to Bangkok. Thanks.

I suspect they must first be translated and then registered (copy/translation) at the MFA in Bangkok - that makes them an official legal document outside of Thailand. Believe the UK offers an alternative method of registration at Embassy which gets them put into a registry in the UK for situations such as this (but is not cheap).

Sounds like you will need to have the documents officially translated into English and then legalized by the UK embassy and the Thai Ministry of Foreign Affairs.

I suspect they must first be translated and then registered (copy/translation) at the MFA in Bangkok - that makes them an official legal document outside of Thailand. Believe the UK offers an alternative method of registration at Embassy which gets them put into a registry in the UK for situations such as this (but is not cheap).

The pension dept in the UK do not want translations because they have to use government translators in England. They just say can send either originals or certified copies. I have emailed them asking much the same question - who can satisfactorily certify these. I would have thought a solicitor or bank official. Take your point about MFA etc, but I think it's just a question of certifying, not registering.