Even flocculants only remove turbidity (particles in suspension....floating) not dissolved matter. They aren't a complete solution either - although a useful tool to add to filtration, heat, chemicals, UV etc. Even distillation (or rain) does not result in 100% pure H2O (otherwise a still would make water, not whisky).
All that said, the chances of finding significant or dangerous contamination in a UK water source are extremely low. Our watercourses are monitored for contamination these days by anglers the rivers authority etc. The most likely toxins to find would likely be cyano toxins from an algal bloom in still water.
If using a decent backpacking fiter or a coarse filter / boil regimen, I wouldn't lose much sleep over UK water sources.