From the ANO:
Instrument meteorological conditions rating (aeroplanes)
(1) Subject to paragraph (2), the rating shall within the United Kingdom:
(a) entitle the holder of a United Kingdom Private Pilot’s Licence (Aeroplanes) or a United Kingdom Basic Commercial Pilot’s Licence (Aeroplanes) to fly as pilot in command of an aeroplane without being subject to the restrictions contained respectively in paragraph (2)(c) or (f) of the privileges of the United Kingdom Private Pilot’s Licence (Aeroplanes) or (3)(g) or (i) of the privileges of the United Kingdom Basic Commercial Pilot’s Licence (Aeroplanes);
(b) entitle the holder of a JAR–FCL Private Pilot Licence (Aeroplane) to fly as pilot in command of an aeroplane in Class D or E airspace in circumstances which require compliance with the Instrument Flight Rules.
(2) The rating shall not entitle the holder of the licence to fly:
(a) on a special VFR flight in a control zone in a flight visibility of less than 3 km; or
(b) when the aeroplane is taking off or landing at any place if the flight visibility below cloud is less than 1800 metres.