I don’t have access to my roof space, is it still possible to install insulation?

I don’t have access to my roof space, is it still possible to install insulation?

Yes. 
If your roof space is hard to access, you can have blown insulation installed by a professional, who will use specialist equipment to blow appropriate insulation material into any awkward space. They may use mineral wool fibre, treated cellulose or polyurethane foam. 
The other option is for the insulation supplier to lift the roof. They may only need to lift a certain section to install batts. Batts are usually a better option than blown-in material.