This is true so far as the problem is stated, referring to "electric showers". But what if the showers are merely power showers that happen to use electric?The shower on the lighting circuit is not acceptable because the smallest shower is around the 7kW mark.
Richie_A: do the showers take in mains pressure cold water and heat it up in their own box? (If so = electric showers.) Or they take feeds from the cold tank and pre-heated cylinder and mix them, using a pump motor somewhere to increase the pressue? (If so = power shower.)