The UK can issue an IDP from both the 1949 treaty (one year validity) and the 1968 treaty (three year validity). The endorsements for motorcycles and scooters are different.
The IDP (1949) has only one category for motorcycles and covers all sizes.
The IDP (1968) has two categories. One for small bikes (scooters/mopeds) and a different category that covers bikes of any size.
When Thailand ratified the 1968 treaty they registered a reservation that says they do not recognize the small bike category.
https://treaties.un.org/pages/ViewDetailsIII.aspx?src=TREATY&mtdsg_no=XI-B-19&chapter=11
Thailand
"Thailand will not be bound by article 52 of this Convention.
"Thailand will consider mopeds as motor-cycles."
Reservation and declaration made upon signature and confirmed upon ratification:
“The Government of the Kingdom of Thailand […] declares that, in accordance with paragraph 1 of Article 54 of the Convention, the Kingdom of Thailand does not consider itself bound by Article 52 of this Convention.”
“[I]n accordance with paragraph 2 of Article 54 of the Convention, the Kingdom of Thailand declares that, for the purposes of the application of this Convention, it treats mopeds as motor cycles (Article 1 (n))”.