If you can access the floor from below (via an unheated cellar/basement) it will be easier, otherwise you'll have to lift the floorboards.
Renewable insulation will need to be protected by a breathable membrane if the floorboards are not well sealed to protect it. See the website of the supplier of insulation you're using (or call their advice line) for advice on what type of membrane would be needed. See below for some links to supplier of natural insulation materials.
225mm of a renewable or mineral fibre type of insulation is a decent amount, with good underfloor ventilation.
Loose fill insulation can be carried between the joists on netting (nailed to floor joists) or on a low-vapour resistance board (for example softboard, a fibreboard bonded by heat rather than glues - this is good for Warmcel insulation made from recycled newspaper).
For other insulation materials (e.g. standard mineral fibre types) it will also be worth looking up the website of the manufacturer (e.g. Rockwool or Knauf) as they'll usually have guidance sheets on how the material should be installed and what limitations there may be.
You can also see some information on the these web sites: