No one is going to guarantee that a water source is safe to drink from. The best you can do is look for reasons that a given water source might be expected to be unsafe. Then, after you rule out as many sources of risk as you can, you make your own determination whether that water source seems safe enough to you.
Personally, I like to use a topographic map to figure out where the headwaters of a given stream is. I try to get water from as close to the headwaters as possible. And I look at the land use of all of the stream upstream from where I'm taking water. Ideally, you would want to know about the historical land use, in case of any sources of groundwater contamination (old underground gas tanks, dump sites of mystery chemicals, mining, etc).
Keep in mind that filters remove microorganisms and undissolved particles. They don't do anything about dissolved chemical contaminants and heavy metals.
Here's a source of water quality testing testing in the UK:
UK Environment Agency Water Quality Archive
The Water Quality Archive provides data on water quality measurements. Samples are taken at sampling points around England and can be from coastal or estuarine waters, rivers, lakes, ponds, canals or groundwaters. They are taken for a number of purposes including compliance assessment against discharge permits, investigation of pollution incidents or environmental monitoring. The archive provides data on measurements and samples dating from 2000.