In either the US or the UK, I don't think that ATC can tactically alleviate the 250 below 10,000' limit where it applies except in the defined cases below - generally you need prior written permission. Here are the references:
UK ANO Section 2: Rules of The Air #23
Speed Limitation
23 (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.
22 April 2003
And the US FARs:
§ 91.117 Aircraft speed.
(a) Unless otherwise authorized by the Administrator, no person may operate an aircraft below 10,000 feet MSL at an indicated airspeed of more than 250 knots (288 m.p.h.).
(b) Unless otherwise authorized or required by ATC, no person may operate an aircraft at or below 2,500 feet above the surface within 4 nautical miles of the primary airport of a Class C or Class D airspace area at an indicated airspeed of more than 200 knots (230 mph.). This paragraph (b) does not apply to any operations within a Class B airspace area. Such operations shall comply with paragraph (a) of this section.
(c) No person may operate an aircraft in the airspace underlying a Class B airspace area designated for an airport or in a VFR corridor designated through such a Class B airspace area, at an indicated airspeed of more than 200 knots (230 mph).
(d) If the minimum safe airspeed for any particular operation is greater than the maximum speed prescribed in this section, the aircraft may be operated at that minimum speed.
I think the US permits speeds above 250kias beyond 12nms from the coast, but I can't find the reference. Anyone?