Once you have the original back from FCDO with its attached certified Apostille, they need :
1. The original document(s) to be legalised, certified by FCDO (UK)
2. A copy of every single page including an apostille page from FCDO (UK)
3. Legalisation application form and a copy of applicant's passport for reference (* * Non-Thai can omit this requirement)
4. A fee of £10per documentis required (can be paid by debit card/contactless or cash) ed to be translated. RTE London have a list of "approved" translators in UK on their website
Applications for document legalisation should be submitted to The Consular Section, Royal Thai Embassy, 29 Queen's Gate (Basement) , London SW7 5JB