4 ideas
To win the noble-born Aurora, he builds a canoe and sails the great inland Lake. Pressed into a petty war, he finds his tactical proposals ignored, then stumbles into the toxic swamp where London once stood and barely survives. He ends among shepherd tribes who make him their leader after his skill with the bow wins their respect, and he sets out to return for Aurora.
Once cultivation stops, succession follows a fixed order. In the first spring, grass and weeds choke the fields. Brambles and thorn scrub come next, then ash, oak and other trees, until within a few decades the countryside is continuous forest. Domestic animals go feral in parallel, with dogs, cats, pigs and cattle each reverting into distinct wild breeds.
When catastrophe strikes, the educated and technically skilled are the ones able to flee, so the survivors who remain cannot rebuild what they merely used. Machines, printing and science decay into rumour and ruin. Society falls back to the organization that raw force can sustain: walled baronies, serfdom and slavery, with literacy as a rare aristocratic oddity.
This lens reads a great city by what it has concentrated and hidden, not by what it produces. When the Thames is blocked and London floods, its accumulated sewage, rot and industrial residue form a black swamp of lethal vapours. The former centre of the world becomes the one place no one can go.