Electric engines are more efficient than petrol engines in converting fuel into mechanical power, thus they are more economical in fuel use. But their electricity has to come from somewhere. The UK electricity grid is powered by a mix of sources: coal, gas and nuclear with a few renewables thrown in. If you make extra demand in order to fuel electric cars, only fossil fuels, which produce emissions of CO2, can provide the extra capacity. Wind turbines cannot turn any faster than wind speed allows, and extra capacity from nuclear power will be possible only when new nuclear power plants are built.