I think the OPs questions has been pretty well answered. Someone suggested that at least in electrics, you could be confident that if you told a customer certain safety features were needed, at least if he went to another electrician they would just confirm this. I have to say, having seen some "Electricans' work and also having seen them discuss stuff in other threads, this is far from the case. I recently saw an advert for a local electrician who was showing off a recent "Fuse Box" (his words) install. I can't remember precisely, but if there was an RCD / RCBO in it, there was only one (so a maximum one for the whole install). I also saw a discussion where some thought it was ok to leave a wylex rewirable fuse CU in situ and add a whole house RCD in front of it, and others who said it had to come out and a 17th / 18th edition CU go in (can't remember how long ago this was). Strictly, I suppose if the house was old enough, it wasn't a requirement to fit an RCD at all.