But on the other hand.... (and I'm afraid I come from a rotary perspective and the rules differ slightly).... if you go for ATPL exams and don't get the Instrument Rating within three years, then you'd have to resit the exams anyway if you did need to upgrade. Certainly for (H), one can upgrade from CPL(H) to CPL(H)/IR by taking the 6 IR exams and not the 13/14 ATPL exams.
Secondly, CPL exams and groundschool will be more expensive as well!
However, there is a cogent argument which suggests that the CPL exams are harder to study since a broadly similar syllabus is covered in fewer exams. For example, the Navigation paper covers both General Nav and Radio Nav. If you are weaker in one of those, you could fail the whole exam with the CPL but pass one/fail one with ATPL.
You need to study LASORS a bit to decide whether the requirements for a full ATPL will be met in the timescales. If not, then you might as well go for CPL. Possibly as an instructor, you might not get to ATPL in the timeframe but then again, as an instructor, you might want the higher qualification under your belt!
Cheers
Whirlygig