Take your pick, Sibton .......... honestly.
Between 450mm and 1200mm for switches and sockets.
When I took a course on the 17th edition regs (current ones) .......... (current!! - see what I did there?) ......... er .................. it was drilled into us that any new installation/new circuit etc should follow these measurements......... however, many Building Inspectors and electricians claim that a rewire, or a bit of alteration during building work, is exempt from this.
So ............ how does one define a rewire? New circuit, or replacement of existing circuit?
Welcome to the wonderful world of building regs. I live on the cusp of two district councils. When dealing with insulation, one council allows my preferred method, while a hundred metres away, I have to use a different method acceptable to them. So it goes ;o)
Local Building Control has the last word.