Cobblers. The current demand for an electric shower is about as much as a domestic supply will take, which is why I said they were available about as powerful as an electric shower and why I said you may as well fill the bath using the shower. There is no point in having another instant heater that is only as powerful as the shower. If you want to fit an industrial three phase heater feel free but the average householder isn't going to.
Do try and keep up or at least read what is said, hint what does domestic mean?