Since the garage is part of the house, do you really need a separate CU for it. You could give it its own ring and lighting circuit from the main CU.
Is there any regulation that I don't know about that says otherwise?
I suppose one advantage of a CU actually IN the garage is that you wouldn't have to come back into the house covered in oil and sawdust - or worse - to reset a breaker.
Also, I only have to run one cable instead of 3.