How Much Would It Cost To Instal A Pump To Increase Water Pressure To My Electric Shower?

[homestead]

I live in bungalow so not a lot of height above shower.

[bhg481]

I'm not sure but if your present system is a combined pump/water heater (the usual meaning of electric shower) then I don't think you can install an extra pump as the water flow then will be too great for the heater to cope.
Your only solution would be to have your shower replaced by a power shower, which comprises a pump in the airing cupboard and uses the water from a hot-water cylinder. If you have a combi-boiler I don't think that is possible as a power-shower needs a reservoir of hot water.
I stand to be corrected on that as I have no personal experience of combi-boilers other than when a friend had their boiler replaced by a combi-boiler they had to have their power-shower removed.

[The Builder]

It wouldn't make any difference to an electric shower, I'm afraid. They just take what they need, and no more.

bhg's idea of a power shower would be best, but it would involve new hot and cold supplies to the shower position. They have a built-in pump.