From what I gather the CAA IR Test is one of the hardest (and expensive) flying tests,so I would do everything I could to increase my chances of passing it.Although you get no time off an IR for doing an IMC there are bits of the IR you can practise with it.Of course once you have an IR you have got to keep it current whilst doing the ATPL`s and it ain`t cheap.
(ps Foghorn still waiting to hear your life story of your journey to a F/ATPL.
)