We have an 'old fashioned' CH and hot water system with a hot water cylinder heated by an immersion heater (in summer) and an LPG gas boiler in winter. Since we're LPG (not town gas) the difference in cost between electricity and gas is probably not all that much.
The choices would seem to be:-
Electric shower - i.e. electrically heated, running off mains pressure water. Our water mains pressure is quite high, a bit over 5 bar, so flow rate would not be an issue. The only issue is how much flow rate the electricity can heat. Disadvantages - Are 10kW showers acceptable? Advantages - quite simple and cheap, instant availability of hot water.
Gravity fed shower - Flow rate limited by gravity, not by heating capacity (until the hot water runs out), but low pressure so not a 'power shower' by any means. Advantages - quite simple and cheap, not power limited. Disadvantages - low flow rate, takes time to get to temperature.
Pumped shower - similar to gravity feed but more 'power shower' with a pump pusing the water out. A bit more complicated. Advantages - good flow rate, not limited by available power. Disadvantages - not instant heat up, will run out of hot water eventually.
So, what do the team think? My inclination is to go for a 10kW electric shower (or are they only 9.5kW now?) as being relatively easy and having 'instant on' and controllability better than the others. The CU will be very close so that bit is quite easy.