21.—(1) Subject to paragraph (2), an aircraft shall not fly below flight level 100 at a speed which, according to its air speed indicator, is more than 250 knots.
(2) Paragraph (1) shall not apply to—
(a)flights in Class A airspace;
(b)VFR flights or IFR flights in Class B airspace;
(c)IFR flights in Class C airspace;
(d)VFR flights in Class C airspace or VFR flights or IFR flights in Class D airspace when authorised by the appropriate air traffic control unit;
(e)an aircraft taking part in an exhibition of flying for which a permission is required by article 80(1) of the Order, if the flight is made in accordance with the terms of the permission granted to the organiser of the exhibition of flying and in accordance with the conditions of the display authorisation granted to the pilot under article 80(6)(a) of the Order;
(f)the flight of an aircraft flying in accordance with the A Conditions or the B Conditions; or
(g)an aircraft flying in accordance with a written permission granted by the CAA authorising the aircraft to exceed the speed limit in paragraph (1).
(3) The CAA may grant a permission for the purpose of paragraph (2)(g) subject to such conditions as it thinks fit and either generally or in respect of any aircraft or class of aircraft.