Have to agree with Last Plumber. I find my system (with a standard, though Part L compliant coil and 72% SAP seasonal efficiency boiler) can only take the 8kW boiler output for around 20 minutes, then the boiler starts cycling. I estimate the total system efficiency to be around the 25-30% mark (useful heat in cylinder vs chemical energy in gas supplied to house).
I used to think that the inefficiency of the national grid (around 35% of chemical energy in coal etc at power station reching my immersion heater) made gas the more environmentally-friendly option, but I think I was mistaken - plus the decarbonisation of electricity in sunny weather due to solar photovoltaics probably makes the immersion cylinder a better option, environmentally as well as in terms of cost.
To get back to the OP's point, the immersion is usually a backup, though it can also be used in the summer months due to historically low efficiencies of boilers running at part-load. It can also be used at the same time as the boiler, for when you want a bath and don't want to wait (2.3kW from the coil + 3kW from the immersion = 5.3W = less time to wait).