Have you thought about an air source heat pump, e.g. the Daikin Altherma? It'll be a lot cheaper to install than GSHP. Rough cost £7-£10k installed (excluding fiddling with your existing heat distribution) and running costs will be less than oil. Expect seasonal (not peak) coefficient of performance of around 3. You should be able to integrate with solar thermal hot water to reduce running costs.
BUT you need to carefully consider the size of your rads. An oil/gas boiler is typically running flow temps of 60 degC but to get good efficiency/ cheaper running costs out of GSHP or ASHP you'll need to be running at 35 degrees or so. If you have the original rads in the house, and have upgraded your insulation a lot they might be ok, but you'll probably have to double them to get the output.
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Quisling