I want to install a shower which will be fed by the rising main and
heat the water electrically. The existing hot water tank is a long way
away so, unless I install an extra one, pre-heated water isn't really
an option.

I would recommend installing a 10.5kW inline instantaneous electric water
heater. This can run to the basin and a standard thermostatic mixer shower
fitted with a head suitable for electric showers. All water in the room
should be from the mains.

Then, if using stored mains pressure water becomes feasible, the
instantaneous heater can be removed and the shower head replaced by one
suitable for a power shower.

Christian.