Simplest and cleanest method:-
38 x 25mm battens screwed through the existing ceiling, 62.5mm K18 insulated plasterboard fixed to battens, tape and fill joints, emulsion, job done…you may want to assess the impact this will have on your existing ceiling height!
Messy ballache of a method:-
Strip existing ceiling lining, insulate in between joists, 12.5mm plasterboard, tape and fill joints, emulsion, job done.