Forgive the 'questions, questions, questions' but I want to be sure about things.
What size are the conductors in the incoming supply cable with the red, white and brown conductors going to the input to the C63A below?
What size are the three white conductors leaving the C63A and heading to the C40A upstairs?
Is there any space inside the upper consumer unit for either an rcd and mcb or an rcbo? If not is there space 'somewhere' very adjacent for a separate enclosure containing these?
Which of the pairings L1-N, L2-N, L1-L2 has the most spare capacity ie has the lowest kW of connected loads.
What does the C16 to the left of the C63A supply? Is it connected across L1 and L2 ie red and brown/blue
If I remember correctly L1 and L2 are red and blue and N is white (except for the three whites between the C63A and C40A.
Have you found your home's earth electrode?
And should to get the picture straight in my mind, apart from C16 below, the C63 only feeds the C40 above? I ask because it looks like may be some reds go into the bottom left of the C63 - see blue arrow in my attachment.