Think about this: You need 25 hours dual to get a PPL. You need 25 hours Dual to get a CPL. If you also need 100 hours PIC - there's only 50 hours left to credit. So why allow you to credit 100? The only logical answer is that the helicopter hours can count towards, the PIC hours.
The problem is that it's a very poorly written regulation, the interpretation of which differs depending on who at the CAA you ask, and generally the person you do ask doesn't understand the question.
I'm amazed that they're still sending that email, because the CAA have already ruled on this one. I've seen an email from the CAA to the head of training at an ATO confirming it. In fact I sent a copy along with my CPL(A) application, which was issued without question with 67 hours fixed wing PIC.