Whether or not it is a legal requirement to fit an isolator, a lot of us "old hands" find it makes good common sense to have the ability to isolate all poles locally for mechanical maintenance. I myself have an xpelair fan which needs cleaning regularly in a small shower room and the ability to keep the light on while I carry out this task it invaluable. I acknowledge that after over 20 years as an industrial electrical engineer I have seen and experienced things which make me somewhat "over the top" regarding safety, but I make no apologies for that.