Will depend on the type of face plates you are fitting. The standard socket outlet is normally ok attached to 25mm box, but the flush flat plates will require 35mm. So if you install 35mm boxes you are covered for any updates of face plates.
35mm boxes also sit perfectly back to back on a 2" noggin centered in a 4" stud wall.
Niggly, but I always make sure the earth terminal is at the bottom - Less stooping when second fixing. And don't use the cable entry right next to the earth terminal. And obviously, with single boxes, make sure the adjustable lug is on one of the sides - not the top or bottom.
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