Well, another theoretical option would be to have a separate, additional mini-CU with an RCD and MCB(s) for the circuit(s) concerned. Whether that would prove appreciably cheaper than a new CU, I don't know.For an RCD: As there are no RCBOs rated at 5A for this CU and not enough space nor busbars for an extra RCD, then it would need a whole new CU.
Kind Regards, John