How much of a pain is it to fit rockwool? - lift current floorboards and refit

Depending on the type of floorboards depends on the ease of removing them. If they are just basic planks this would be better. If it is tongued floor boards you have a problem. Old floorboards were fitted using cut clasp nails these can rust overtime however they are quite substantial. If the boards are tongued you would have to work plank by plank working out which direction the tongue is. If you just crowbar them up they will split the groove on the next board. If you have standard boards with a bit of careful effort they should come up ok. Then de nail boards and joists. Fitting rock wool between joists is fairly straight forward it will be fine around central heating pipes. Check the type you buy and what you want to achieve ie draught proofing or sound deadening.

use a spray insulation a bit dearer but will be the easiest way to put it under the floor boards

If the floor boards are 150 yrs old you will damage them a lot, What about dropping the ceilings instead you can get lovely flat ceilings after installation of rockwool and then reboard tape and joint or skim, Now if you have lath and plaster ceilings then this could be messy work.