So, just surface fix the back boxes direct to the wall (i.e. no need for chasing in) and your d&d plus skim will finish flush.
However be prepared for the plaster thickness to increase where pipes etc have been run down a wall.
Our local builders merchant used to sell dry lining door casings at 5 3/4" wide. These incidentally turned out a few mm less than the stated size.
I would always to argue that if you were to fix a light switch back box either side of a 100mm thick wall then you would need a casing that was 150mm thick and that their standard casings would require a glued plant in order to comply.
If you want fancy ultra-flat or screwless sockets, then you will need to fit 35mm backboxes, and therefore recess them into the wall by 10mm.
Otherwise, the 25mm will be fine.
Light switches also 25mm - although 16mm can be obtained, they are generally useless.
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