The polio in sewage is an issue because it is of the type that comes from live vaccine used in other countries. That means that people in London but vaccinated elsewhere. are carrying the live virus (but have antibodies)
However, because there is an increasing number of people with live virus, that increases the chance of it spreading and mutating. If it mutates, the dead virus vaccine we give children in the UK may not be as effective.
They have the same issue in New York and there has been one case of a person being affected.
So the authorities are playing it safe and boosting immunity.