Take up the floorboards adjacent to the external wall. Hoover out all the dirt. If there are any gaps you can actually see, fill them with sand and cement mortar. Small cracks and gaps you can fill with expanding foam (I only use the pink fire grade now). It is much easier if you use an applicator gun - look for a Fischer at about £15 on ebay, and a can of gun cleaner. The foam grips better and hardens faster if you mist the surfaces with a water sprayer first to moisten them. It will not stick to dirt.
Then, stuff mineral wool between the joists, tight against the wall, the joist and the floor above. This will stop most cold air movement. I strongly recommend the brown version treated with Ecose which does not shed irritant dust and fibres. It is made by Knauf but widely sold as an own-brand. The packaging will mention Ecose. Don't use the yellow irritant fibreglass.
Bay windows are often badly finished and air may be blowing under or between the tiles.
If you have the energy to take up more of the floor, do the gap against the party walls next. this may also reduce noise transmission and the passage of smoke and odours, so use plenty of pink firefoam where any joists go into the brickwork.
Brickwork that is out of sight between floors, or was intended to be plastered, was often built to a very poor and gappy standard.
If you ever take the whole floor up, consider laying mineral wool over the whole floor, especially if you are in a flat or the room below is unheated. It muffles the nois slightly and will cut heat and draughts. The benefit is not as great as doing just the edges of the room, so I wouldn't bother unless the floor is coming up anyway. You must not block any airbricks - clean them out to remove cobwebs, dust and rubbish and ensure a clear flow of air under the ground floor.
However, if this is a ground floor you are talking about, it is worth putting mineral wool between the joists under the entire floor, or draughts will blow up through any gaps. This will be easier if you can access from below. Because mineral wool squashes to give a tight fit, it is IMO better for draughtproofing than rigid plastic foam, which is seldom a perfect fit.
After Grenfell I would not use plastic foam insulation.
