Keeping water and food stations clean. Making sure feed is fresh, clean up spills to ensure feed doesn't mold. Cleaning up excess poop. Having good ventilation in your coop.
All of these things are pretty good preventative measures to help keep your flock healthy.
There is an extremely long list of medications that may be used to treat an equally long list of conditions and illnesses.
Is there a particular disease, condition or medication that you are interested in? You are located in the UK so most antibiotics/medications are by prescription only. Getting a yearly fecal float to check for worms would be a good idea.
I have to ask - are you writing a book?