In-depth heat pumps
This is a good article.
The Coefficient of Performance (CoP) depends on the outside temperature so instead you should look at the Seasonal Performance Factor (SPF) which the installer should calculate. This calculation demonstrates how the heat pump should perform given the average temperatures at your location and other details such as the size of your radiators.
To transfer more heat into the house with lower temperature water you need larger radiators or underfloor heating. So smaller radiators need higher temperature water which costs more energy to run.
Installing a heat pump might need planning permission in a conservation area.
For hot water heat pumps usually only heat up to 55°C but the water needs to periodically exceed 60 to kill bacteria. It will need an immersion heater to do this.