Not really sure of any other option than an electric. You haven't said how big the your tank and immersion or how it's configured.
Unfortunately with a standard set up you'd probably only get half a decent shower with a mixer type before the water ran out and then have to wait an hour or so for the water temperature to recover to finish off!
A modern 10.8kw shower is pretty good you know.
The only other alternative is an instant gas water heater, and consequently serious money and installation.
You can't fit a mixer shower direct off cold mains and tanks fed hot water, you would have to fit a cold water tank too.
This is water regulations and health and safety - because they say if the cold mains was to be turned off either to the house or to the street for any reason then you could be scalded as the hot water would still flow from the tank.
If your tank is on the same level as the shower - which I'm sure it will be then the pressure will be very poor.
Your best bet is to go for an electric shower - Mira are good and the pressure will be good.
Yes you can fit a pump on any low pressure system.
But I say again, with only an immersion heater to heat the water, a pumped shower will only give enough hot water for about a 5 - 10 minute shower. You would then have to wait 2 - 3 hours before any purposeful hot water was available!
Consider also, you would need to heat the water on the daytime tariff, which is astronomically expensive compared to other fuels (including Ecoomy 7)!
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