Hi Mike,
If you already have a water heating system, then that would be the way to go.
Tee off from the flow pipe on the radiator above floor level, enough to allow you to fit a thermo' valve (non directional) to this side of the system. Drop from the valve through the floor to a snake of 8mm or 10mm mini-bore pipe under the floor boards (8mm should be enough). The spacing of the loops would roughly be about every foot (300mm). Try to keep as close to the tops of the joists as possible, roughly again, would say about
30 mm down for your drill holes, and remember, when laying the boards down again, to mark on the walls where the pipes are just in case. Try to stay away from exteriors walls as much as you can, say about a foot (300mm) again should be OK. Then it's just back to a tee connection in the return side of the system, which can be under the floor if you want, and the jobs done.No matter what type of covering you put down, you'll always have the heat rising through it. It should keep the place lovely and cosy for your dainty little feet. Under Floor Heating does the business.