[GordonS]

Plus, don't GSHPs need quite a lot of land for the piping? Houses in the UK are built on the smallest possible plots of land :(

It's normal for temperatures in Scotland to dip below -5C over winter - do ASHPs not work at those temps?

[fest]

For horizontal designs yes- the rule of thumb is the brine coil area should be around 3x the heated area. However, the vertical bores don't need much area, just expensive drilling work.