1980s, solid floors, I'm assuming no insulation, but need to look into the regs at the time to get a better idea. Any ideas where to find that info? I don't know where to start!
EPC, speak to neighbours, drill a core sample… profit?
If you don’t mind losing a bit of head height, you could always overlay with 100mm PIR then pour screed over wet UFH. Or just get nice, cheap & effective big rads…
Do you know what your current solid floor build-up is? What kind of house?