The inconvenience of a single main RCD was demonstrated to me when I first moved into my present house. The element on the shower decided to short out one night while I was using it, leaving me to stumble about in the dark trying to find a towel and then fumble my way around the place to the main panel to restore power.

Dave,
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What is the area of these small boxes you are using? They sound extremely small. Do you have large devices like GFCIs?
Sorry, I missed that question first time around.

Have a look here for our typical flush mounting boxes.

Deeper boxes are available as you can see, but this is the box used for a twin socket (duplex receptacle) in the average house:
[Linked Image from tlc-direct.co.uk]
In English units, that box is approx. 5" long, 2.7" high, and only 1" deep.

Yes, GFI-style outlets have become available in recent years. This one is supposed to fit in the same 1" deep box as a regular outlet.