As far as the Regulations are concerned, try Regulation 554.3.3, which covers 'Water heaters having immersed and uninsulated heating elements' (no, NOT electrode boilers, we are talking 'immersion type' heaters!).Electric showers contain a small tank with an instantaneous immersed water heater in it, and hence come into this category.
It states " The heater or boiler shall be permanently connected to the electricity supply through a double-pole linked switch which is either separate from and within easy reach of the heater or boiler or is incorporated therin".
If you then check Regulation 701.512.3, erection of switchgear, control gear etc etc, you will see it states that
"The following requirements do not apply to switches and controls which are incorporated in fixed current-using equipment suitable for use in that zone or to insulating pull cords of cord operated switches".
Those are the Regs that probably cover your question. Other factors like isolation, safety etc have been mentioned by other contributors already so I wont add any more. Hope that helps
The Oracle