About the only extra in the IR is airways navigation. The IMC Rating does not entitle you to fly airways or in IMC in Class A airspace - although in VMC, SVFR is permissible with the relevant clearance in, for example, Class A CTRs. But not for en-route airways flying.
For the IR, you will probably spend hours and hours grinding round the NDB hold - this seems an obsession of most FTOs!
The main problem is that you will need to pass much more stringent CAA exams, many of which have precious little to do with Instrument Flying. You will need to sit them at a regional test centre, do your flying training at a FTO and take the IR Skill Test with a CAA Staff Examiner - plus the CAA still use the IR Skill Test as the final filter for commercial flying suitability, rather than as an assessment of safe IF skills as the military do. Whereas for the IMC Rating you can do everything at a RF and take the test with a suitably qualified PPL/FE.
There is some light at the end of the tunnel though. AOPA started an initiative to have the JAR-FCL IR(A) brought closer to ICAO requirements and to have the theoretical knowledge requirements eased.
Few aircraft used for the IMC Rating Test have the luxury of RMIs or stable gyro compasses, so the specific handling skills are different to those needed for the IR. Battling with a crosswind NDB approach in a less-than-young Cherokee with a primitive ADF and fixed card RBI with only a DI for guidance is a lot more difficult than just towing the tail of a RMI whilst referring to a nice accurate HSI....
The IMC Rating is often denigrated by self-important IR holders who have never held an IMC rating. It is an extremely useful rating and should not be thought of as merely a 'get me out of this' rating!