Common low pressure sodium and white LED street lights are about the same in efficiency: around 140 lm/W. LED street lighting pollutes the night sky with light that astronomers find impossible to fully compensate for, impacts wildlife (and probably not in a good way), inhibits low-light vision adaption in drivers and pedestrians, and gives public areas all the attractiveness of an operating theatre. All this for a dubious promise of savings after tens of millions of initial investment from each local authority.
The estimated UK population is clearly growing; net migration has been in the positive hundreds of thousands nearly every year since 1998, even exceeding 500,000 in 2022. Why would this trend not only stop, but reverse for long enough to cause a significant population decline?
I'm sure that continuing to grow the population at an unsustainable rate will solve this. Need those new consumers to make the line on the graph go up, no matter the consequences.