It would seem logical that ATPL groundschool providers were also CPL/IR groundschool providers by default, just that most of them can't be arsed to offer the courses because the majority of market demand is for the ATPL sausage machine and most, but certainly not all, CPL/IR candidates aspire to be airline pilots. After all, the CPL/IR exams share the exact same content with the ATPLs, but the latter have some additional syllabus items (global climatology, transatlantic planning etc..). Whether that is the case in reality is another matter; the CAA are never ones to let common sense get in the way of red tape and stupidity.
The format of the exams is also different. The CPL flight planning exam for example combines performance, mass & balance, so it's quite a wide scope which requires a fair bit of revision. CPL and IR both have a single nav exam, with the former being mostly VFR and the latter more IFR. They also both have a met exam, with slight differences (no adiabatic lapse rates etc in the CPL, so IR met is harder). Anyone who's undertaken a full ATPL groundschool should have no problem passing the CPL and IR exams.
That said, if you've done ATPL groundschool it may be more convenient to do the ATPL exams as you've been trained for them. To take the CPL and IR exams will require different revision schedules, and it's possible that the inconvenience of that will outweigh the fact that CPL/IR exams are slightly easier. Plus you'd be 1 exam better off - ATPL is 14 exams, CPL is 9, IR is 7. IR Human Performance credits for the CPL equivilent, so that makes 15 versus the ATPL 14.