As it happens my floor is very leaky, and lifting those boards would be very challenging. So working from below is the right move which brings me to insulation.
Ignoring draughts completely for now, If I calculate the impact of 100mm of rockwool on my floor, my living room heatloss (when it's -4 outside) goes from 1.72kw to 1.40kw. So a 320W. saving. But that's worst case.
Drops to 190W if I assume an average of 5 degrees outside.
Works out about £39 per year (double for current oil prices of 7p per KWh).
So for materials only we're looking at 13 year payback with normal oil prices. So meh, may worth it, maybe not.
Mine will be much high higher return as it will also solve the draught issue at the same time. Air changes are hard to calculate, but it might double the return.
But the real driver isn't cost for me.
With the heating on, you can be warm sat on the sofa, but sit on the floor and it's freezing. You can feel the cold air being drawn up. Not fun for the kids, and limits how we can use the space.