What would most likely happen if londoners started putting household waste in the thames again?

The Thames would be declared biologically dead

Explanation:

In the last 50 years, the thames river has seen a significant increase in aquatic life and biodiversity due to laws preventing dumbing of householf waste and sewerage in the Thames.

If Londoners started putting household waste in the Thames again, this biodiversity would be reduced very quickly the river Thames would be declared biologically dead.  This would have a significant impact on the health of londoners living near the river resulting in outbreaks of disease such a cholera.