You are quite correct, but anyone who expects there to be a regulation backing up every last detail of good design will be sadly disappointed. I don't think you will find a regulation in BS7671 stating that light switches are required in or near the room that a light is installed in. However I imagine you fit switches for lights that you install, rather than relying upon the MCB. Some items of electrical design are down to common sense, functional operation, logical positioning etc. Such that an untrained, non skilled person should be able to use the installation easily during its everyday operation, and isolate any items requiring disconnection for maintenance or for rapid isolation in the event of a fault. e.g. smoke, burning, melting, arcing, sparking, visible, audible or electrical smells coming from an item of electrical equipment. These are some of the basic functions of local isolation. Also the possibility that a fault inside the shower could cause an RCD to trip may need the neutral to be easily isolated to allow an RCD protecting multiple circuits to be reset. A single pole MCB would not provide this isolation. I think most manufactures suggest a double pole isolation switch with 3mm separation contact gap.
Doc H.