An electric tankless an abusive abomination, putting a LOT stress on the local power distribution grid.
To achieve adequate performance the instantaneous power draws for a whole-house tankless anywhere in the UK would exceed 20,000 watts (a single shower at a 25C temperature rise takes about 15,000 watts) and may even exceed the capacity of the service connection to the house in some instances, and puts undue stresses on the transformer, and creates power frequency & voltage disturbances on the local distribution grid (especially when turning on or off.)
By contrast an electric cylinder type water heater typically draws less than 5000 watts when heating, can support much higher flows (for many minutes before the stored heat in the cylinder is depleted.)
The electric tankless discussion should rightly be in a new thread, not tagged onto a thread about a propane tankless water heater.