For canberra and others, in the UK Rule 23 is the one that matters for speed.
(1) Subject to paragraph (3), an aircraft shall not fly below flight level 100 at a speed which according to its air speed indicator is more than 250 knots unless it is flying in accordance with the terms of a written permission of the Authority.
(2) The Authority may grant a permission for the purpose of this rule subject to such conditions as it thinks fit and either generally or in respect of any aircraft or class of aircraft.
(3) Paragraph (1) shall not apply to:
(a) flight in Class A airspace;
(b) VFR flight or IFR flight in Class B airspace;
(c) IFR flight in Class C airspace;
(d) VFR flight in Class C airspace or VFR flight or IFR flight in Class D airspace when authorised by the appropriate air traffic control unit;
(e) the flight of an aircraft taking part in an exhibition of flying for which a permission under article 61 of the Order is required, if the flight is made in accordance with the terms of a permission granted to the organiser of the exhibition of flying under article 61
of the Order, and in accordance with the conditions of a display authorisation granted to the pilot under article 61 of the Order; or
(f) the flight of an aircraft flying in accordance with the ‘A Conditions' or the ‘B Conditions' set forth in Schedule 2 to the Order.
In simple terms, if you are flying in airspace where other pilots may be flying VFR and aren't getting some for of ATC surveillance and traffic information, the limit is 250 kts.
Outdated rules? Yes probaby. A bit too simplistically summarised above? Yes probably, but that's what it seems to boil down to.