Maybe you can just tie a drawstring to the body without the pull tab? If not you'll need a reverse slider, to replace it you cut off the top stopper, remove the old slider, fit the new one and crimp a new stopper in place. Some more inf0 here: https://www.gearaid.com/blogs/learn/zipper-slider-replace-how-to
This seems to be a set of slider and stopper but it doesn't say so explicitly. Maybe someone else has a better lead for a UK source. I'd send you one of my sliders but I don't have the stopper you need so it wouldn't really solve your problem.
Maybe you can just tie a drawstring to the body without the pull tab? If not you'll need a reverse slider, to replace it you cut off the top stopper, remove the old slider, fit the new one and crimp a new stopper in place. Some more inf0 here: https://www.gearaid.com/blogs/learn/zipper-slider-replace-how-to
https://www.shelbyoutdoor.com/product_info.php?products_id=2925
https://www.shelbyoutdoor.com/product_info.php?&products_id=1864
This seems to be a set of slider and stopper but it doesn't say so explicitly. Maybe someone else has a better lead for a UK source. I'd send you one of my sliders but I don't have the stopper you need so it wouldn't really solve your problem.
https://www.ebay.co.uk/itm/293527773976