Article 49 Height keeping performance—aircraft registered elsewhere than in the United Kingdom
(1)
Unless otherwise authorised by the appropriate air traffic control unit an aircraft registered elsewhere than in the United Kingdom shall not fly in United Kingdom reduced vertical separation minimum airspace unless:
(a)
it is so equipped with height keeping systems as to comply with the law of the country in which the aircraft is registered insofar as that law requires it to be so equipped when flying in any specified areas; and
(b)
the said equipment is capable of being operated so as to enable the aircraft to maintain the height keeping performance prescribed in respect of the airspace in which the aircraft is flying, and it is so operated.