I'll second that. While the IMC rating is a CAA (not a JAA) rating, and therefore is not valid overseas and not necessary as part of the JAA frozen ATPL, it does give a good headstart with the IR - and the ATPL groundschool. Also, it helps to do hour-building in marginal VFR in the UK when you would otherwise look at the wx forecast and think: "mmmm... maybe not today". As has been mentioned, the minima are higher than for the IR (plus 200', with a minimum DH of 500' AGL), but I found that having done the IMC rating helped a great deal when getting to grips with the IR. By then, I had actually practiced NDB/VOR holds, precision and non-precision approaches, etc.
Cheers
P.S.: another piece of good news is that the IMC validity is 25 months, and that it is renewed by that amount every time you renew your SPA/IR.