If you get a pass in all 13 ATPL(H) ground exams, they are valid for issue of a CPL(H) and an IR(H) for 36months. You then have 7 years from the date of your last IR renewal to get the required hours for the issue of an ATPL(H).
If after 36 months you have only taken a CPL(H), your theory credits for an IR(H) have lapsed, and along with it your ATPL(H) credits. To now get an ATPL(H) you must retake all 13 exams (and obtain an IR(H))
If you do CPL theory from the start, those exams are only valid for the issue of a CPL(H).
If you feel
1. You will never want an ATPL(H); or
2. You will be unable to get an IR(H) within 36mths
then CPL theory is an option. But be aware it firmly closes a door. With a little bit more work, you have kept your options open for an ATPL.