They are available in France and used extensively in renovation work with exposed internal stone walls, must have installed over 100 and never had a problem, trouble is I can't remember the brand, and can't find them now on my old wholesalers web site, but I am sure it was Quinetic.
Just...
As above you will probably find the originals where oriented the same way and your replacements are upside down to each other, or visa versa. I used to work on L2 = Blue and go from there.
So do Hager: Flush Mounted Consumer Units | Hager UK - https://hager.com/uk/products/residential-distribution/consumer-units/flush-mounted-consumer-units
I think your main problem is going to be the height of cut out in the front panel, needs to be checked against manufactures dims for RCBO's, MCB etc: may have to look for a new front panel, never know MK may still use the same dims between the fixing screws and position of DIN rail?
Without going into too much detail get hold of this book: https://www.amazon.co.uk/Practical-Inspection-Certification-Electrical-Installations/dp/1138613320/ref=asc All the C & G question and model answers are in it.
There is nothing statutory that says you have to take any course to enable you to work, so whatever you do is not outside the law, saying that the C&G are a respected source of learning and would be adequate defence in the very, very highly unlikely situation that you had to prove competence...
I your OH really wants small diameter lights have a look at decking lights seem to come in all sorts of small diameters, IP should not be a problem either.
Why do you think you need to pay for an 18the edition course rather than reading up what changed and inwardly digesting, all someone on the course will do is read the 18th edition at you.
These could help: LEDlite Hole Conversion Plate - Chrome | LEDlite (LTHCP Chrome) - https://www.tlc-direct.co.uk/Products/LTHCPC.html
Quite often used to use Bonding to put a plasterboard batten over a hole from above and then a plasterboard circle cut from an off cut Bonded to the batten, just...
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