The procedure for making a document legal in Thailand is to have the document signed by a Notary then obtain an Apostille from the Foreign & Commonwealth Office (FCO) before submitting it to the Royal Thai Consualte.
The FCO has two offices, one at Milton Keynes (postal centre), and one in London which offers a Premium Service for business users. For FCO charges click here.:
Royal Thai Embassy Contact Details:
Royal Thai Embassy
29 Queen’s Gate (Basement)
LONDON SW7 5JB
Tel: 020 7589 2944