"(If we went the combi route we would get an electric shower). "
For why? Combis do brilliant showers as you've got mains pressure hot and cold. Electric shower are pretty dreadful by comparison.
In this three bed detached house with four hot taps and four occupants..parents and two very tall kids, the combi is just great. Its a six year old Vokera Linea 28, and just through the wall from it in the bathroom is a Trevi Therm shower. I have a programmable thermostat (Danfos) and Thermostatic Rad valves and the system is really very good and easy to live with,and been no trouble since we put it in.
Get a decent quality combi with a good hot water flow rate and you are laughing.
Alfredo