The Royal Aero Club issued aviator's certificates between 1910 and 1914 which permitted the holders to take part in their competitions and aerial demonstrations. They held no particular legal privileges as I understand.
After WW1 at the Paris Peace Conference in 1919 the International Air Convention was signed by many of the participants. Article 12 stated:
" The Commanding Officer, pilots, engineers and other
members of the operating crew of every aircraft shall, in
accordance with the conditions laid down in Annex E,
be provided with certificates of competency and licences
issued or rendered valid by the State whose nationality
the aircraft possesses."
This task was given to the Air Ministry in the UK which was formed initially to set up the Royal Air Force, and later to the ARB and the CAA.