To teach for the IMC rating you must have either a valid IMC or a valid IR. You must have an unrestricted FI rating and 10 hours by sole reference to instruments. Complete the applied IF course with a FIC instructor and pass a test with a FIE.
If you hold a FI(R) rating you would need a valid IR have 800 hours flight time IFR or 200 by sole reference to instruments and complete the IRI course.
Details in AIC 110/1999
http://www.ais.org.uk/Uk_aip/pdf/aic/4W369.PDF
Application form to remove the restriction to your CPL(R) attached to AIC 47/2000
http://www.ais.org.uk/Uk_aip/pdf/aic/4W019.PDF