Thanks again!
It may well be the CBIR I end up doing, but in my situation with no IMC rating or IFR experience the difference is probably academic, the only change being I may get a few hours shaved off the course if I'm ready for test before the normal 50 hours for the SEIR. From what I've been told, the CBIR is identical to the full IR (and appears exactly the same on the license) apart from the training route. If I've understood correctly, the HPA exam is only for those who haven't sat either the IR or ATPL exams?
Would having the CPL before starting the IR be useful in any way other than simply being more experience handling the controls and staying within tolerances?
I guess I could just look up the tolerances for the IR skills test on the CAA site and use these as a guide - but is there anything else to consider in terms of flying ability when making the decision to do the IR first?