We insulated under our floorboards at our last house (coming up from the cellar) and it was ok, but still pretty drafty. We have recently moved and this time we had the gaps filled whilst they were being sanded back and there is no breeze or drafts at all. I'd say this is by far the best option.
We did this and used that insulation stuff that comes in rolls in silver packaging (sorry don't know technical term). OH fide it himself but he was able to get under the floor. Its made a massive difference