RC is quite correct. Putting a bit more flesh on what he says:
A UK (but not a JAA) CPL gives you similar instrument privileges to holding an IMC rating but with no renewal/revalidation required. If you want to fly a single piston (SEP) or twin (MEP) then you must have a current SEP or MEP rating. SEP ratings are valid for 2 yrs, MEP for one. If your rating is out of date (by up to 5 yrs) then you need to take a Licensing Proficiency Check with a FE PPL or CRE. This is just a bash through the standard GH items.