yeah, it's possible to retrofit a ground source heat pump.
the advice i got was to do things in stages
1. double the loft and wall insulation everywhere
2. Lay underfloor heating 1st and run it from my current oil fired boiler
3. install a wood burning stove to replace the existing open fireplace (this involved installing a back boiler water tank too)
4. fit solar panels to the roof and use that to heat the water
5. install a windturbine and a system to sell it back to the grid if i generate more than i use.
6. install ground source/air source heatpump, removing the boiler completely and running the underfloor heating and hot water from it. connect the solar panels up to a bank of batteries and use that to supply electric along with windturbine (selling excess back).
the wood burning stove was really only because i wanted a fire in the lounge for winter - more aesthetic than anything else!!! But at least it could be practical.
but the above process takes a long time and is messy and i don't want to live in a building site!
if i ever build from scratch, the ground source pump, underfloor heating, solar and wind turbine will all be 1st in - along with as much insulation as can be fitted into the place
i'll also have all the plumbing set up to feed a large water tank that can be used to water the garden/plants.
and for the garden i'd have a Walpini!!
http://www.bensoninstitute.org/Publication/Manuals/Walipini.pdf