I'd recommend doing the CPL first.
Its a logical progression to finish off all the VFR training before switching to IFR training, fellow students who complete the IR first have told me they struggle with the switch back to VFR flight and have to be re-disciplined into getting their head out of the cockpit for the CPL training.
Another issue is partial panel training, depending on which JAR state you decide to train for your IR you may only cover partial panel flying during a full UK 25 hour CPL.