The ANO is
here and the relevant stuff is on page 340 of the PDF.
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.
I struggle to see what is in there which makes it other than 1500m, in Class G.
Obviously if you fly outside the UK, then the local airspace rules may require a better visibility. But are there any such countries (now)?