ICAO Annex 1
1.2.1 Authority to act as a flight crew member
A person shall not act as a flight crew member of an aircraft
unless a valid licence is held showing compliance with the
specifications of this Annex and appropriate to the duties to be
performed by that person. The licence shall have been issued
by the State of Registry of that aircraft or by any other
Contracting State and rendered valid by the State of Registry
of that aircraft.
Note.— Article 29 of the Convention on International Civil
Aviation requires that the flight crew members carry their
appropriate licences on board every aircraft engaged in
international air navigation.
Art 62(1) renders the licence valid. The CAA does not confirm items of law, that's a matter for a Lawyer.