Try Article 225 of the ANO:
(1) Subject to paragraph (2), an aircraft registered elsewhere than in the United Kingdom must not fly over the United Kingdom for the purpose of aerial photography or aerial survey or for the purpose of any other form of aerial work unless:
(a) the CAA has granted permission to do so to the operator or charterer of the aircraft; and
(b) any conditions, to which such permission may be subject, are satisfied
(2) This article does not apply to an aircraft:
(a) registered in an EEA State
(b) registered in territory to which the Air Navigation (Overseas Territories) Order 2013 applies
(c) registered in the Isle of Man or Bailiwicks of Jersey or Guernsey
The default position, therefore, is that aerial work is not permitted but the CAA may grant exceptional permission. It is left up to the CAA to decide what conditions any permission may be subject to and, historically, the condition applied to permission to conduct flight instruction is that it has been restricted to the owner/co-owners or to pilots employed by the owner in the case of private exec aircraft.