In reality if you are removing the old circuits and installing new cabling, then that would sound to me like a new circuit rather than an old one. I doubt very much that the new cable replacing the old circuit would be a true representation of what was originally existing, therefore I would conclude in saying this was not a new circuit would be a little cheeky !
The CU in the garage exists.
It has two circuits which exist.
The cable of these circuits can be replaced - not a new circuit.
The circuit itself can be extended - not a new circuit.
Whichever is done first is irrelevant.
After the work there will still be two circuits.
They will be longer and consist of new cable but neither of them is new.
Abiding by the law is admirable.
Thinking of ways to be constrained by it when it is reasonable not to be is silly.