I'm not sure if this has been discussed before, but the FCO in the UK have announced that the Thai authorities require that UK public documents are legalised before they can be used for official purposes in Thailand. From 1 June 2018, documents originating from the UK to be used for official purposes in Thailand will need to go through the legalisation process in the UK. 

Once these documents have been legalised by the UK Legalisation Office, they must be legalised by the Royal Thai Embassy in London, then legalised by the Ministry of Foreign Affairs in Thailand before they can be used in Thailand. In particular this change will affect educational and other documents required for work permit and visa applications. Examples of such documents include degree certificates, professional qualifications, job references, police checks, birth certificates, death certificates and marriage certificates.

 Please note that some documents may need to be certified by a solicitor or notary public in the UK before they can be submitted to the UK Legalisation Office. If you need a notary contact the Faculty. 

I've seen no mention of this before but it seems that it will affect a number of expats that have to provide proof from the UK, and I assume other countries, to support their applications, I don't know how wide ranging this will be.

 

https://www.gov.uk/guidance/notarial-and-documentary-services-guide-for-thailand?utm_source=031554b1-f21c-465a-9038-f4700184be2e&utm_medium=email&utm_campaign=govuk-notifications&utm_content=daily

It has been that way for sometime now. Not sure why they are saying tomorrow.

This PDF link posted on the embassy website is dated September of 2015. 

https://assets.publishing.service.gov.uk/government/uploads/system/uploads/attachment_data/file/712469/Legalising_sig_stamp_Sept_15.pdf

Before then they would allow you to do a statement that it was true and correct that immigration and others would accept.

Edit: Also from the updates at the bottom of the page you posted a link to.

• 24 September 2015 Updated to Services provided elsewhere section
• 17 September 2015 Changes to Legalising a signature or seal section

 

I gleamed and followed that information from the FCO site way back in 2004 when the legalisation office was still at the Old Admiralty Building in London.

I employed a Notary who had the document legalised by the FCO, then the Thai Embassy for a total of £90 all inclusive.

At that time it didn't have to be legalised by the MFA in Thailand. The Thai Embassy vignette on the document was acceptable.