It is uncommon to see anything below 6A in domesitc systems, but a few suppliers do ones below 6A but usually C/D curve.1 x 3A Smoke detector circuit (shows the age as it was a 3A 60898 which I haven't seen before)
It probably did not start that way! Sadly it was clearly not a designed process of what and how things would be usedWhat beggars belief is that someone thought it was a good idea to run those two sub mains from a 40A MCB in the first place!
Having two sub-mains back to the main DB at least would allow better visibility of total loads if nothing else can be done.