I am mystified by this since the first Siamese embassy to England and France took place in 1684, 331 years ago. In England the embassy had two audiences with King Charles II and presented him gifts from King Narai. Details of this embassy, in French, are kept at the Missions Etrangeres in Rue du Bac, Paris.
The first diplomatic mission to England in modern times was to Queen Victoria in 1857, which also proceeded to France to present its credentials to Napoleon III at Fontainebleau. Siam established its first permanent embassy in London in 1881 and in Paris in 1884.
It would be good if the Foreign Ministry could hire a student in Paris to go to the Missions Etrangeres and make copies of the 17th-century documents. As for the colonial or modern period, records abound in both French and English archives and publications, besides our own in the National Archive.
Sumet Jumsai