FWIW where I live in London the issue is that the local drains going along residential etc streets are overwhelmed. So there are various overflow locations etc

FWIW where I live in London the issue is that the local drains going along residential etc streets are overwhelmed. So there are various overflow locations etc where when the sewers are full to bursting and overflowing back up into people's toilets, they don't and instead overflow into local water courses. We're miles away from the super sewer, so they'd need to improve the capacity from our house all the way for that to work. We have a particular issue with just surface water (not sewage luckily!) where I live. Usual finger pointing game between the council and Thames water and us, but the word from the guy from the council is that the surface water drains are just too small for modern weather systems, so when we get a very heavy downfall we have water being forced up and out of the drains where our guttering empties into, leading to about 2 or 3 inches of water around the entire house before it drains away into lower land in the garden. I don't fully believe him that the pipes are too small (it rained in the past too!) - I suspect it is just partially silted-up/blocked/collapsed somewhere nearby after over 100 years of neglect, but of course Thames water have no money nor impetus to fix it. We've had non-return valves fitted in the pipes on our property (at a fairly large expense since we had to dig down) to try and abate our house getting flooded when it rains. So far so good....