Hi Rachel,
The fundamental thing you need to know is what is your source water like. If the water coming out of your tap is of reasonable quality, not too hard, not too soft, has decent amount of carbonate, some electrolytes. Then you only need to remove any chlorine/chloramine that has been added to treat the water. Any half way decent "water ager", "tap water treatment" will do the job (if you want real UK brand names, I'll look them up for you). The only exception is if you know that your water have a heavy metal problem then you would need something that will neutralise that as well. But most of the time, town water is strictly monitored, we hope.

Beyond that, you are starting to get into the smoke and mirror stuff. Okay, you can get stuff to promote slime coat but who is to say that their slime coat is not of the best quality already? You can add vitamins and minerals. All these should be in a well balanced fish diet. Anything that you can think of that is suppose to make your fish healthy, live longer and happier can be added to a bottle.
The most fundamental thing that the fish need is good, clean, dechlored water with no ammonia, nitrIte and minimum nitrAte. Stable environment in terms of pH, temp and oxygen level. A healthy balanced diet and those fish can fight off 99.9% of all pathogens.