When it comes to suspended timber floors, the main heat loss culprit is convection heat loss (aka draughts, air leakage) and not conduction (which insulation addresses). Therefore a hugely effective (and relative cheaper) measure to consider without huge disruption internally is to seal the floor from wall to wall with an air tightness membrane (the current floor covering would need to be temporarily removed or replaced). This prevents the cooler outside air from coming up uncontrolled through the floor (or visa versa with warm internal air being lost through it) and does not change the ffl generally.
Doing this alone throughout the house could have quite a large impact in the overall heat loss equation.
any reason you couldn't put the membrane on top of the existing boards, then a floating floor on top with an insulated underlay? Assuming you can manage the increased floor height. The room I work in has suspended floor and is super draughty, this is my plan for it, as ripping up the boards is a much bigger job and I'd probably want to put laminate down anyway.
EDIT - actually I see that's what you meant by "lift the floor covering"