

That is not correct - the surface drainage system should be regarded as separate from the sewage system, even though both run under the roads. There is the surface water system and the foul water system. It’s true that in some places surface drainage may go into the sewage system but that is the exception rather than the rule. Surface drainage is usually designed to move as rapidly as possible into nearby fresh water to prevent flooding.
Surface drainage water is allowed to drain freely into water courses, rivers and lakes, completely untreated. The sewage system is for contaminated water (from toilets and sinks etc) and is designed to go to treatments plants where it SHOULD be treated. It is true that that treatment is not happening, and when there are storms the sewage system can be overrun with water companies currently getting away with dumping contaminated sewage into the rivers which is a scandal.
There are 9.8m people in London. If everyone was pouring the dregs of their coffee into the surface water drainage it’d be an environmental mess.
Contaminated fluids including dregs of coffee belong in the sewage system, not the surface water drainage system. This is literally the same as pouring coffee into a river or a lake - that’s where the surface water system is designed to run to directly, untreated. In London, that’s the Thames receiving that directly.