16mm boxes are a real pita at switches, I had one where the customer had just decorated, then decided to have some new dimmer switches fitted, they rang me when they discovered the switch wouldn't fit onto the existing box, fortunately I was able to peel the paper back enough to hack out the old box, make the hole deeper and get a deeper one in.
The one I always hated was the way councils wired their houses, all in singles, neutral looped at the light and live looped at the switch, no permanent live at the fitting, always a nuisance when fitting those fan/light units that were popular some years ago.