What socket metal back boxes do I need? 25mm for sockets, 15mm for light switches? Correct? Or could I use 25mm for light switch also?

Box depth is best decided by the make and type of accessory being used + the room needed for wiring.

Ordinary simple light switches mostly use a 16mm deep box, but some are best fitted into a 25mm box, or even deeper, especially if it is a dimmer switch, multi-gang switch or you need room for a number of cables to it (Many dimmer and metal plate switches will not physically fit into a 16mm box)..

Most ordinary sockets can be fitted into a 25mm deep box, but again you will need to check with the accessory being used and the room for wiring. (Many metal plate / slim plate sockets need a 35mm deep box).

There is no harm in using a deeper box than strictly necessary and is often done for making the fitting of the accessory easier with less risk of trapping cables etc. The additional cost is minimal compared to the time taken to remove and change an insufficient depth box.

Hope this helps.

The size of a box is in part determined by the number of wires, fittings and straps that will be contained in it. A box that is bigger than it has to be is SO much easier to close up! If in doubt, go big!

Whenever possible limit the number of wires/cables in/out of a box. Two sets is never a problem. Three is still good, but 4 or more and you might have to go with an extension ring or a double wide box.

I'm in the US, and don't think in those terms, but 25mm is about 1" fractional, and seems way too small, and 15mm is like tiny. Not for home wiring, but maybe fine for like a guitar amp. Please rethink, and update.

UPDATE: Go with ELECTRIC LES, who's been there, 'done that. Lacking the word "depth", I had width in mind. Here, that's more like standard, deep, or shallow/pancake. Default is to standard, deep when the box is also needed as a junction box, shallow when there's not enough cavity depth for more. 'Outa here.

You must have very different methods in UK, because nothing in USA would fit in a 15 or 16 mm deep box. Wirenuts would barely fit!

Our "standard" box is closer to 50 mm deep!

Our boxes also need to be so many cubic inches (volume) depending on how many conductors are in the box.

hh