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
(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).