I think these problems are often found with retro fitted systems. We are lucky that ours was designed for the property when it was built. For maximum efficiency you need to have them running continuously, but a thermostat in every room, as we have, means that the heating only comes on when the temperature drops below what you’ve set. For heating water we find the immersion heater is better, just programme it to come on and off to suit your needs and boost if necessary,
I don’t agree that this will have to be afforded one day ilovecheese. A vast amount of the country’s housing stock is reliant on oil or gas heating. The cost of converting every such property, not necessarily producing good results, is unimaginable. Much as I love my air source heat pump, I wouldn’t want a retro fitted system imposed on me if I had oil heating (which is the only alternative out here in the sticks unless you have the space to be able to comply with the regulations for having a tank of calor gas).