Whilst not accepting the arguments put forward by Chopjock, he does have a correct understanding of the (somewhat liberal and soon to be replaced) regulations of the UK.
Sadly, they do not agree with those provided in the pamphlet referenced by Talk The Torque; they are also
not in compliance with ICAO Annex 2 - Rules of the Air:
Flight outside controlled airspace
28.—
(1) Subject to paragraph (6), an aircraft flying outside controlled airspace at or above flight level 100 shall remain at least 1,500 metres horizontally and 1,000 feet vertically away from cloud and in a flight visibility of at least 8 km.
(2) Subject to paragraphs (3), (4), (5) and (6), an aircraft flying outside controlled airspace below flight level 100 shall remain at least 1,500 metres horizontally and 1,000 feet vertically away from cloud and in a flight visibility of at least 5 km.
(3) Paragraph (2) shall not apply to an aircraft which—
(a)flies at or below 3,000 feet above mean sea level;
(b)remains clear of cloud with the surface in sight; and
(c)is in a flight visibility of at least 5 km.
(4) Paragraph (2) shall not apply to an aircraft which is not a helicopter and which—
(a)flies at or below 3,000 feet above mean sea level;
(b)flies at a speed which, according to its air speed indicator, is 140 knots or less;
(c)remains clear of cloud with the surface in sight; and
(d)is in a flight visibility of at least 1,500 metres.
(5) Paragraph (2) shall not apply to a helicopter which—(a)flies at or below 3,000 feet above mean sea level;
(b)flies at a speed which, having regard to the visibility, is reasonable; and
(c)remains clear of cloud with the surface in sight.
(6) Paragraphs (1) and (2) shall not apply to a helicopter which is air-taxiing or conducting manoeuvres in accordance with rule 6(i)