From google...
"The IMC Rating is a national rating that can be added to a UK issued pilot licence. It is designed to enhance the privileges and instrument flying skills of a PPL holder that will enable a pilot to fly in weather conditions below the minimum requirements for a basic PPL holder. However, it is not a substitute for a full Instrument Rating, which is required for flight into controlled airspace.
Flying training and experience requirements may be found in the CAA LASORS publication, downloadable from
www.caa.co.uk.
The ground examinations for the IMC Rating include questions on the planning and execution of a typical flight under instrument flight rules (IFR) outside of controlled airspace. The exams must be completed within 12 months and the flight test within 9 months preceding the date of application for the rating"