Has anything changed recently?
My understanding (checked with the CAA some years ago) is that you cannot pay an instructor to teach you for a rating on a Private Cat aircraft (or a Permit one), but can pay an examiner to do a test for renewal of a rating on one.
Apparently (I've not done this), if the instructor is prepared to instruct for free, that's OK on a Private Cat - but if you then pay him way over the odds for ground school, they might come to a conclusion...
I'm due for my IMC rating renewal shortly, and would prefer to take it on the group aircraft.