> I do find it odd air-to-air source heat pumps are not considered more often in the UK

Could be due to housing density and the reputation for noise, though I'm not sure how well-deserved that really is.

Mostly I just think it is a mismatch between the age of housing stock (and the pre-existing heating solutions) with incentives.

If I were building a new house I wouldn't bother with a gas supply and would use a ground-source heat pump combined with underfloor heating - but retrofitting that to my 150 year old house is never going to be cost effective versus a boiler.

For residential developments, the government should just mandate no new domestic gas supplies and be done with it.