I think that the CAA are correct, the ATPLs being at a different level and with a couple of extra subjects thrown in, cover you for the IR(H) knowledge.
JAR FCL 2.470 shows the table of exams to be passed for which level you have chosen.
The CPL(H) exams will only ever allow you to be awarded a CPL(H) and no IR(H) until you take the 7 IR(H) exams. This is why for some people the Dragon / Atlantic option is best if they never want to fly IMC. The others are generally best advised to go along another route Bristol, Cabair etc and do the ATPLS, thus getting all the exams out of the way at the beginning, but starting the clock from the Month of the last pass.
I have a CPL(H)/IR and will probably never hold an ATPL(H) as I have no multi crew time. I did however complete the ATPL(H) writtens prior to getting an IR.
JAR FCL does use the word applicant, so you can probably start the flying training without having passed all the exams (sensible or not is another matter), you just can't apply for the rating.
I am not an IRI so may be talking b*ll*cks but it seems pretty clear cut to me.
Gary