You do not need a CU of any kind, terminate the cable into a metal clad socket and add any others from that, use a 3A switched FCU for lighting.the other end of the armoured cable (in the garage) will enter a mini consumer-unit with RCD (its terminated at present before I reconnect it) in the garage before being distributed to power the lighting and a plug socket in there. Would it be acceptable to earth the armour at that end?.
While not ideal, connecting the armour at one end only could be compliant.