Is it ok to put insulation between roof rafters?

You can also add rigid foam insulation under the beams, which adds an R value and eliminates the formation of thermal bridges through the wooden beams.

Inside these hatches, I noticed, due to the wind and the cold, that there is no insulation at the bottom of the tiles.

Well, now it's not covered by building regulations, but apparently, it's not your loft either or you wouldn't have this problem, so yes, insulate with that, it will stop the cold to a certain extent, but not as good as the Celotex or Kingspan 120mm thermal plate and, of course, rock wool is much cheaper. You must leave a space in the back of the insulation between the membrane and the insulation to allow air to circulate through the roof space.