The main places to insulate are the ones with the biggest gaps – so open windows, under doors, and gaps under and around windows
Think about insulating your loft hatch to stop warm air being drawn up and out. Cut insulation to fit the hatch but make sure the edges are well draught-proofed.
Homebase have a handy article on loft insulation here.
If you’re doing building works, it’s worth laying a layer of under-floor insulation to avoid heat escaping through the floor.