Is it safe to use a metal light switch faceplate on a non-RCD protected circuit?

80A could be one of the reasons, as the MCBs total more than 80A.
Indeed, but I would think that there must be literally millions of CUs in which the same is true, but my view is that one needs to apply some common sense (call it 'diversity' if you wish'. Indeed, in a different context, you very recently asserted (and I agreed with you) that it would be extremely rare for domestic circuits (like sockets ones) to be 'fully loaded' (such as to being conductors up to their maximum permitted temp - i.e. it would be incredibly rare for two domestic sockets circuits to amount for anything like 40A of load - so, even for the very brief period that a shower was being used, the total on that RCD would almost certainly be well under 80A.

As I have implied, I really don't think that many people would be worried about adding another 6A's worth of MCBs to that RCD
In this example the supplier fuse appears to be 60A so that may not be a problem if the fuseholder label is correct.
I'm surprised that you say that, given how 'strictly' (I'll avoid the p-word :) ) you view such things. In the (extremely unlikely) situation in which all of the MCBs were "fully loaded", a 60A fuse would allow that total current (104A) to dlow for an appreciable period of time, very probably longer than the duration of the average shower.
Others are that it's a type AC RCD, which can't be used for anything other than resistive loads
Again, there must be literally millions of UK CUs in which the RCD's are Type AC.
moving the lighting there will result in most of the installation on a single RCD, see division of installation
There is obviously appreciable variation in interpretation of that (very vague) reg, but nearly all of the arguments about it seem to relate to 'the danger' of having all lighting circuits protected by the same RCD - but that would not be the case if one of the OP's lighting circuits was moved to the RCD side - although, ironically, that change would increase the risk of that one of the lighting circuits will be lost in the event of an RCD trip!

In any event, as I always say, someone who is worried about all lighting being on one RCD (not the situation we're discussing) here but does not have emergency lighting is either daft or lives in a different world from mine (in which loss of {all} mains-powered lighting is far more commonly due to a power cut' than to an RCD tripping)!
and the 9mA leakage current per RCD is likely to be exceeded, and may already be.
Again, I can but refer you to 'millions of installations' out there, and the experience of myself and others around me that this is not necessarily anything like as big an issue as some seem to think. In any event, what matters is which, and how many, 'leaky' loads are plugged into socket(s) circuits, and one could, in theory, get a high total leakage with things just plugged into one circuit. The fact that there are two socket circuits protected by the OP's CU is therefore notr necessarily relevant and, if there had only been one, even you could not have complained of adding an extra 6A to an existing 'total of MCB ratings' of 72A!

If Mr Jobsworth were to raise the points you have, I could not argue about the points, but I could/would question whether they were 'sensible' in relation to, or representative of, the real world. However, my main worry, which I've often voiced before, relates to your tendency to make dogmatic assertions which I often feel are likely to 'mislead'. This started because you wrote ...
Nothing can be added to the RCD side in that consumer unit.
... and it is my belief that many people (who did not know better) would take that to mean either that (a) to do so would be physically impossible (which clearly is not the case with the OP's CU) and/or (b) that it is something that no electrician would consider doing (and I very much doubt that is the case).

Kind Regards, John
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