I'm doing a major redecorate (long overdue). I am replacing the switches and sockets with flat-style ones, in my case the Chelsea range sold by TLC. I had some problems fitting them, so I thought I might be able to save someone else time by describing possible problems.
You need to make sure the back boxes are level and deep enough before you start to decorate.
There is a bezel on the back of the switch or socket that fits snugly into the box, so you can't use the movable screw lug to make fine adjustments to make them level. If you have top and bottoms lugs you will need to bend them flat, though the makers do warn you about that.
Because the devices don't stick out from the wall they go a bit further back into the box. One of my switch socket boxes was the shallow variety and I had to dig it out and replace it with a deep one after I'd finished painting. Sigh!
The rear of the device is also a bit bigger vertically and horizontally than some older types, so might foul on earthing terminals at the back of the box.
So, don't wait till you've decorated before getting the sockets. Try them out before you start. It's almost certain you need to change the boxes. They look really smart though.
Peter Scott