Also have drunk from streams around the UK for years without puritabs or a problem – think you have to be fairly unlucky.
The one time I did have a problem I didn’t drink anything but didn’t realise that upstream of the river I was swimming across to go surfing was a sewage plant which, when it rained heavily, I paraphrase “open storage tanks to purge the extra liquids to ensure the tanks are not over-filled, this can include raw sewage”. Nice. A days vomiting and squits followed by a helicobacter infestation and 3 weeks on 3.5g of antibiotics per day and I’m fine 🙂 Still drink from streams.