The Core

 Utility Week
https://utilityweek.co.uk/gas-connections-new-homes-banned-2025/

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"Gas is four times cheaper than electricity, and homes with a gas supply are one and a half times less likely to be in fuel poverty."

Plans to ban gas heating in new homes must be put on hold
https://www.gmb.org.uk/news/plans-ban-gas-heating-new-homes-must-be-put-hold

I didn't realize UK used RU gas. I would have guessed North Sea and/or Africa.

Besides their cost, air-to air heat pumps have 2 main problems;

1) Their size and layout doesn't fit well in high-density urban dwellings such as large apartment buildings.

2) Their efficiency tails off quickly at ambient air temps below 40f/4.4c.  We do well with them here in the mid-South but less temperate places like (much of??) the UK would be relying on auxiliary electric heating elements a significant portion of the cold months. 

>All their units are made by Mitsubishi.

 
Mitsubishi has very high seer units, 24-ish, so that makes sense in that locale.  Down here, the sweet spot for cost vs efficiency is 14 seer.

Those "suitcase" heat pumps (Mitsubishi, Panasonic, others?) also have more compact outdoor condenser units ....which is why you see them plastered all over the sides of apartments and condos in Spain, Italy, & SE Asia.

So, yes, they work in some places but they have a long way to go to be able to replace gas, particularly in existing buildings.