Whopity already answered the exam question, but just to add if you did ATPL groundschool from the onset, got the CPL(A) but then decided not to do the IR it would not affect you having a CPL(A) at all.
If it then went over three years and you suddenly decided that you did want an IR, you would have to do the groundschool exams again.
But then if you only do the CPL(A) exams from the onset and decided you wanted an IR after, you'd be in the same position of having to do loads more groundschool.
If you're thinking £30k is a lot to justify, then why are you looking to add another £15-20k on an IR you will probably never use? It's only valid for a year, and very expensive to keep up if you're only using it occasionally, and if you're teaching PPL(A) students then you won't be flying in cloud much at all. Sure, there's the odd day when you'd punch up through some cloud to teach a basic lesson like climbing/descending once you're "VFR on top" but it's not a necessity to be able to do this, and it only works on days when you know the cloud tops are quite low, no point having to climb to 8000' just to start the lesson.