1. Find the real drafts before you buy anything
Guessing wastes money. The gaps you feel with the back of your hand are rarely the biggest ones, because a strong leak in a cold hallway can feel identical to a weak leak in a warm room. Spend twenty minutes mapping instead.
Pick a cold, breezy evening. Close every window and external door, then turn on the kitchen extractor fan and the bathroom fan at the same time. That drops the pressure inside slightly and pulls outside air in through every gap you have. Now walk the house with a lit incense stick or a thin strip of tissue taped to a pencil. Hold it still for three seconds at each spot and watch for the smoke or paper to bend.
Check these in order: the bottom and hinge side of every external door, the meeting rail of sash windows, the frame edges of casement windows, the letterplate and keyhole, loft hatches, the gap where floorboards meet skirting boards, and any disused fireplace. Mark each hit with a strip of masking tape and a number so you can build one shopping list instead of making three trips.
Two things to note while you walk. Air bricks, trickle vents and vents serving a gas appliance are there on purpose — leave them alone. And a room that feels cold but shows no moving smoke usually has a heat loss problem, not a draft problem, which means insulation or a radiator issue rather than tape.