You only get a 12 hour dispensation off an approved IR course if you are adding the rating to a CAA licence (making the multi IR a 43 hour course). If you are adding it to a JAR licence you get absoultely nothing off the course.
You should have no problem using your own aircraft for the CPL and IR. The aircraft will need to get a one-off dispensation from the CAA if you wish to use if for the test, and I think ideally it should be approved if you wish to carry out training in it, although this may not be legally required. You just need to find a school that is happy to train you in your own aircraft. You're probably more likely to find this from a smaller FTO - I can give you the name of a couple from the North of England if that's where you want to do it.