S0d lifting the boards and laying branded insulation sticks.
You could easily drill a few holes and then blast polyurethane foam into them.
It's the same stuff the sticks are made out of (like kingspan), and the resin / blowing agent is cheaper when bought and blown in than the preformed boards; kind of ironically. But you will need A LOT of it, not a few squirty cans. Enough googling and you will find the guys doing it in big cylinders. They're usually messing around with boat hulls or spraying entire barns.
You'll want gaps for the foam to come out of, or it'll bend the centre of the floor up a little. You may want to check it with a level before and after, as I've seen the cans do it with PVC frames and cause the window to catch as it closes / opens. A number of injection holes will help with that, so it has somewhere to expand out of. Metal casters do the same thing, they pour in through one gate and let it flow out another to be sure the cavity is full and has somewhere to expand to if it needs to (aluminium / zinc / iron shrinks as it cools though).
Using expanding foam also seals every single gap in the void. People will drop in sticks of predone foam, but then leave gaps where it's sawed to size for the timbers. Those gaps will let drafts through, and have massive impact on the effectiveness of the insulation. You NEED those sealed to match the lab results, which is what a blown foam will do. I have a roof done with preformed sticks and downlights in the plasterboard over it. If I take a light out, I can FEEL the air moving and see things moving around the hole. The expensive insulation is there, but the gaps are too. Foil insulations say "You HAVE to tape the edges of this" and that's the reason why, drafts destroying it's potential.
You'd need to work out where the timbers are, ROUGHLY how big the volume is under there (poke sticks down and measure then length, then work out the volumes) and then the foam guys will tell you how much resin / blowing agent it'll need to fill. Pick a cylinder set slightly larger, blown slightly more in and let it squirt out the holes, then chop it down and proceed as normal.
I bet the Germans guys are onto this!
e.g. Saw a stick down, bang it up, then foam the last few mm to get a draft tight seal. The UK is MILES behind the darn Germans when it comes to building efficiencies, it's embarrassing. They're working on buildings that have essentially 0% loss from insulation / air con / heating / drafts, using the same things we are. The war is still on, and it's building efficiencies (a very positive war)!
