Apparently it is legal in the UK for flights outside CAS; CAP393 extract below:
Instrument Flight Rules
32.—(1) For flights within controlled airspace rules 33, 34, 35, 36 and 37 shall be the Instrument
Flight Rules.
(2) For flights outside controlled airspace rules 33 and 34 shall be the Instrument Flight Rules.
Minimum height
33.—(1) Subject to paragraphs (2) and (3), an aircraft shall not fly at a height of less than 1,000
feet above the highest obstacle within a distance of 5 nautical miles of the aircraft unless—
(a) it is necessary for the aircraft to do so in order to take off or land;
(b) the aircraft flies on a route notified for the purposes of this rule;
(c) the aircraft has been otherwise authorised by the competent authority in relation to the
area over which the aircraft is flying; or
(d) the aircraft flies at an altitude not exceeding 3,000 feet above mean sea level and remains
clear of cloud and with the surface in sight and in a flight visibility of at least 800 metres.
(2) The aircraft shall comply with rule 5.
(3) Paragraph (1) shall not apply to a helicopter that is air-taxiing or conducting manoeuvres in
accordance with rule 6(i).
Quadrantal rule and semi-circular rule
34.—(1) Subject to paragraphs (2) and (3), an aircraft in level flight above 3,000 feet above
mean sea level or above the appropriate transition altitude, whichever is the higher, shall be flown
at a level appropriate to its magnetic track, in accordance with Table 1 or Table 2, as appropriate.
(2) For the purposes of paragraph (1), the level of flight shall be measured by an altimeter set—
(a) in the case of a flight over the United Kingdom, to a pressure setting of 1013.2
hectopascals; or
(b) in the case of any other flight, according to the system published by the competent
authority in relation to the area over which the aircraft is flying.
(3) An aircraft may be flown at a level other than the level required by paragraph (1) if it flies:
(a) in conformity with instructions given by an air traffic control unit;
(b) in accordance with notified en-route holding patterns; or
(c) in accordance with holding procedures notified in relation to an aerodrome.
(4) For the purposes of this rule ‘transition altitude’ means the altitude which is notified in
relation to flights over notified areas.
No mention of descending below MSA etc.
JJ