Your initial comment was that the average UK house isn’t suitable for heat pumps. This is incorrect, in the vast majority of UK homes (including the poorly insulated) they can be heated sufficiently by a heat pump.

Don’t spread misinformation. Some installers have fixed prices regardless of the number and size of radiators. There’s even solutions out there where a hot water tank can be fitted into a standard kitchen cupboard.

You keep mentioning high temperature units which are less efficient, they are only needed in very niche installs or used by poorly trained installers.