Ground Source Heat Pumps using borehole arrays are a proven technology and have been very widely used for many decades in most, if not all, northern european countries.
This technology has been relatively slow to be widely adopted in the UK, primarily for two simple reasons
1. Cheaper to install energy saving methods have been effectively subsidised and promoted e.g solar panels
2. The installation of ground source heat pumps with boreholes requires a degree of technical knowledge that must be acquired by experience. Not to mention a significant investment in plant & machinery