Electric boiler for on demand hot water

I have 2 bathrooms. Small bathroom has a electric shower which is fine. There is a old immersion hot water heater gravity fed. No pressure in hot water taps. Bathroom plus kitchen. I am thinking of changing it to a electric boiler on demand hot water. My mains pressure is ok so is it ok for one shower plus 3 hot water taps in the flat. Would the pressure be ok or it would also be on the lower side if shower and a tap is being used simultaneously. My flat heating is electric so the boiler is not needed for that purpose just on demand hot water supply? Please advise what is better the above or a new unvented hot water tank for hot water supply. I am also thinking of installing a jacuzzi tub so which system is better?