I'm sorry for your loss
Finding medications can sometimes be difficult. Even though we (in the US) have access to more, some things are a little harder to get due to the way of the world right now.
I try to keep a few things on hand. One is Amprolium (Corid, Coxoid) another is poultry vitamins (for you in the UK Net Tex Poultry Nutri-Drops would be good). I also keep a few first aid items too (ointment(s), vet wrap, etc.). I don't use these things too often thankfully, but it's nice to have them just in case.
Just something to think about - I would not necessarily go out and buy everything at once, but you may want to consider adding an item to your chicken first aid kit here and there when you can.