At a guess, an IMCR will give you single crew IFR privileges in UK airspace Classes D,E,F,G whereas your multi crew JAA ATPL will give you no single crew IFR privileges whatsoever.
Whether, once you have your ATPL, you could knock off a quick IR checkride in some piston twin to get a single crew ME IR (which is way better than an IMCR because it gives you worldwide IFR privileges in any JAA-reg single crew SE or ME plane) in less time, is a good question.
An IMCR is rarely done in 15hrs to the required standard. 25 is more like it.