The second CU is for the heat pump which is indeed inverter-driven. From memory (not at home at present), the MCB is a 20 Amp 6kA C Curve. This CU also feeds the garage for some tech gear I work on which is power hungry, and will be also be used for a future 3.5kw PV array (still to be installed).
I was under the impression that a Type A RCD was the minimum you could use (although Type B is the prefered device).
I didn’t have any input in this installation as I wasn’t at home when the work was carried out. It took a year to have the whole installation ripped out and redone after an audit. To this day I am still finding bodge’s to the wiring.