yes, if you do have a modern combi it will have a lower cost per shower but the cost of a new boiler is very high in comparison. My own boiler is now 40 years old and i am reluctant to replace it as the total cost of maintenance has been under 40 pound for 2 thermocouples and a thermostat. It has a high efficiency as the heat exchanger is cast iron and it recovers heat from the exhaust gases.
If i had put an electric shower in my house it would have cost a lot less but i gained in water quantity for an average 4 minute shower.
Energy saving is an interesting subject and i can see no reason that new housing has electric panel heaters costing 5 times the price of wet radiators heated by a gas boiler.
In 1977, someone had decided a housing development here should be heated by panels that warmed the plasterboard ceiling. We were doing this work for the builder. When the customers moved in, they just thought the bill was wrong. Just the living room panels used 5kw and it was full price electricity not off peak. Most of this system was removed in the first year and never installed anywhere again.
Back then, i saw the electricity bill when the cost was about 6 pence a unit and a quarter bill was 270 pounds on one house. Ouch !