Bose, You are correct but the IMC is valid for a year longer than the IR. Since the priviledges are basically the same for practical flying purposes (as long as you fly in the UK, out of airways and who goes down to 200' anyway) it takes the pressure and cost off renewing every year (and this thread was about economising).
I also find it bizarre that an IR can be renewed in a sim but an IMC needs to be done in a plane.