I'm not sure how they'd work to be honest. The doubts I have about them:
* How would the heat be moved around the house?
* Obtaining permission to modify the exterior of a multi-occupancy dwelling is extremely difficult.
* You'd still require a source of hot water, reducing the already small savings over an efficient combi-boiler.
Based on the estimated savings of the Energy Saving Trust, it'd take around 26 years to pay off a £3k install that replaces a modern combi-boiler[0] and that's without the cost of using electricity to provide heated water. As recently as March of this year Which claimed most houses would have paid an extra £80/year if running a heat pump[1].
They could be a good choice for completely new installs and people stuck on resistive electric heating if they can find a way to disperse the heat evenly. I'd bet a lot of money that the majority of properties with electric heating are rentals, good luck convincing a landlord to pay for anything.
[0] https://energysavingtrust.org.uk/advice/air-source-heat-pump...
[1] https://www.which.co.uk/reviews/ground-and-air-source-heat-p...