Are you multi -rated yet if not go and do as a PPL student as this can be a cheaper way than as a commercial student and use eny cash you save to fly the multi befor you start the IR.
The hardest thing i ever did in flying was the IMC rating because it was all new to me the IR is just a matter of polishing the skills.
the good thing with the UK/JAA IR is that you will know what is comming next in the test and so can practice the test more or less as you will get it ,when i did the FAA/IR it was much more of a test of piloting skill as you did not know what was going to happen next and had to be able to think on your feet.