As far as I am aware it is not mandatory to have a consumer unit, I have raised the question many times as to where it says we must use type tested stuff in premises under the control of an ordinary person, and not found where it says that. So if a non BG transformer is fitted into a BG distribution unit all it means is it is no longer a consumer unit.
Many consumer units have the German standard (DIN) rail mounting, so you could fit another make, although some have only half a DIN rail, and some times the hole where the MCB/RCBO would normally come through the cover so you can operate them does not align, and also where bus bars are used, often they also don't align, also the temperature inside the unit may go out of spec, so theory says should not be mounted in the consumer unit, however an inspector doing an EICR would need to be rather pedantic to code a non permitted item inside the CU.
I know when working in Sizewell power station some items where installed and then a wall built around them, I asked about maintenance, and was told easy, just knock the wall down, so even behind a wall still accessible. We talk about maintenance free, and with a batching plant I have as part of the maintenance schedule tested the torque on all the terminals. However I have never seen a domestic home with a maintenance schedule, so what is the point in making it accessible for maintenance, if that maintenance is never carried out? Even with the main consumer unit, I have never seen anyone test the torque on the terminals after first installed.
However one must be aware people do feel for things under floor boards and in ceilings, so any part in the void will need to be in an enclosure of some sort to stop anyone in error touching live terminals, or mouse or spider, since don't want the RCD tripping.
We have to use some common sense, and in hind sight, when I fitted this lamp
View attachment 300251 fitting the relay inside the lamp was not the best move, as to get to it, even if it is only behind 5 knurled nuts, as to remove all the dangling bits will take some time. It may never go wrong, but if you make it hard to access, you can bet your life you will want to even if just to fault find, and nothing has gone wrong.