I'm pretty sure there's nowhere in the mainland UK that just dumps all its raw sewage in the sea or in rivers - everywhere on the mainland was retrofitted with sewage treatment a while back. Now, those retrofits aren't perfect and sometimes raw sewage still overflows due to storms for example but we don't just routinely discharge all the sewage into the sea. There are still places in Spain, Ireland, and some other EU countries that haven't managed to build sewage treatment yet and actually do discharge all their raw sewage either into the sea or into rivers leading to it. (Yes, this is almost exactly the opposite of the narrative pushed by the British media.)
Gibraltar apparently doesn't have sewage treatment because they have some salt-water flush system for toilets due to fresh water historically being so expensive there and they seem to have had difficulty getting a treatment system that works with that.
Boil-water notices due to cryptosporidium and other nasties in drinking water are somewhat common in some places in Ireland. The difference in water quality and hardness between places that aren't very far apart is pretty striking.
Gibraltar apparently doesn't have sewage treatment because they have some salt-water flush system for toilets due to fresh water historically being so expensive there and they seem to have had difficulty getting a treatment system that works with that.