Ordinary chlorine and iodine tablets kill bacteria and viruses but, per NaTHNaC, parasites like Cryptosporidium and Giardia “are not always destroyed by these products”. That is the gap that catches people, because Cryptosporidium is exactly what upland British surface water offers.
Chlorine dioxide is a different product despite the similar name – the CDC has it killing Giardia and “somewhat effective” against Cryptosporidium, while still advising you filter first. Read “somewhat effective” as narrowing the gap rather than closing it, and see the water page.