Article 93 Height keeping performance - aircraft not registered in the TerritoryI1
United Kingdom
An aircraft registered elsewhere than in the Territory must not fly in notified reduced vertical separation minimum airspace in the Territory unless—
it is equipped with height keeping systems so 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
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.
An aircraft need not comply with the requirements of paragraph (1) where the flight has been authorised by the appropriate air traffic control unit notwithstanding the lack of compliance and provided that the aircraft complies with any instructions the air traffic control unit may give in the particular case.