You need the most insulated house you can get due to the low temp differential of the ASHP hot water vs the room temp.

I have a friend in Sweden with one on his small summer house that works well, but is insulated to Nordic standards.
The outside units can be loud , they are effectively the same as those split AC units you see on building with a fan etc.

Ground source heat pump will work better if you have the land to bury the pipes. I guess on new build you have that chance.

I know Octopus do a special tarriff. Called Cosy Octopus
Day rate:

25.25p / kWh
Cosy rate (04:00 - 07:00 & 13:00 - 16:00):

12.37p / kWh
Peak rate (16:00 - 19:00):

36.62p / kWh
Standing charge:

47.85p / day

I would get the help of a specialist installer to tell you what makes sense. I'm keeping gas for as long as possible. Lets us know the conclusions.

My PV & battery set up is running at a 7 year pay back & that should reduce to 6 or less when I move to a smart tarriff. Cost was under 8k for a 3.6kWh array & 4.2kWh batteries. We have generated 80kW in April & imported 10.6 kW & we exported 8.9 kW.

JohnV6
2022 CX Plus Four
2025 Renault Scenic Alpine etech "Yvette"