The 10mm cable is approx 10meters to where it is then jbd. There would be an approx run of 5meters from this jb to garage where the garage cu is going.Not sure if I've understood you right here.
40A fine if clipped direct and why the RCD, surely the circuit now becomes a distribution not a final and RCD protection should be provide in the garage CU and the 10mm/40A should be moved to the non-RCD side.
Of course all depends on garage location, cable length, type and route.
Thanks