Hi Auchy
Its not CPL theory + IR theory = ATPL theory credit.
Its holding a CPL(H) with an IR(H) which was gained under ammendment 3 of JAR FCL 2 which gives you the ATPL(H) theory credit.
You should be able to get a credit for HPL but you need to ask the CAA for this.
I would get the CAA to confirm in writting what youve asked to avoid any problems in the future.