My 1960s retrofit already has concrete floors so I’ve not done this personally, but as no-one else has offered an opinion, here’s mine.
Unless the space is too deep to make it practical, I’d fill it in. so you can get a properly insulated floor with some thermal mass. and get decent airtightness. Building control people get very difficult about airbricks, airtightness and suspended timber floors so getting rid of it is both technically and ‘politically’ better.
Hopefully the space is just right for 200-300mm of insulation. I default to XPS rather than PUR underground just in case it ever got soaked (flooding), but that’s carbon-intensive too, so if you’d prefer to avoid that foamglass agreegate is very green (but also expensive when I looked). https://www.mikewye.co.uk/product-category/brands/geocell-foam-glass/ You’d just pour that in instead of the hardcore, then put a floor on top. Lithotherm UFH floor blocks directly on top of the foamglass is one way to do concreteless solid floor: https://www.backtoearth.co.uk/product/underfloor-heating-tiles/ I’ve not tried it myself but I’ve seen one and it looks like a reasonable plan. Backtoearth are great so I’d certainly have a chat to them about it.
You could take the view that having this nice space to fill in with some insulation is a bonus. Dealing with an existing concrete floor is a pain because digging it up is a big job and insulation on top is technically limiting, especially with UFH.
