Two Instrument Ratings are available to UK pilots. There’sthe full IR and the IR(R), with the second R standing for Restricted, based on the UK’s old IMC Rating (Instrument Meteorological Conditions) devised to help pilots get out of trouble should they find themselves in IMC.
Training for the IR has been transformed in recent years and a common route is the Competency Based IR. It’s an Instrument Rating aimed at private pilots, rather than airline pilots. It allows private pilots to fly in cloud, in any class of airspace, including airways. You can take off in any visibility conditions and fly a Class 1 ILS descent to a decision height of 200ft and RVR of 550m (two crew) or 800m (single pilot). That’s plenty for most pilots.
The IR for private pilots has seven subjects of Theoretical Knowledge, plus a minimum of 40 hours of IFR flying time, including 25 dual, of which at least 10 must be with an Approved Training Organisation.