Thanks for sharing your findings which I can relate to. Air source heat pumps are still quite expensive. Given that they are similar in operation to fridges which have been around for a while, heat pumps could be much cheaper. But then, I suppose, they are on an early stage in the learning curve and installers have to learn installing them, too, possibly charging a premium.

Perhaps making one step back, the question could be asked: "How much are we willing to pay to make home greener?" by other measures. Heat that does not get lost does not need to be replaced, i.e. invest in insulation and refurbishment. The UK has a large, poorly insulated housing stock and a tradition in building houses similar to the ones in the 1930ies using similar designs. Why not go a few steps further: https://peter-wurmsdobler.medium.com/changing-the-face-of-a-country-for-the-sake-of-energy-efficiency-e3d564f9a0b