Trees can be a hazard. Roots and branches need to be monitored, for tripping or falling risks, or the risk of the tree falling over. Trees need to be trimmed, especially where they intrude into problem space - cables (very rare in London), double decker buses, people's property, and so on.

The database needs to be built.

Also as someone else pointed out I got my maths wrong (I blame no calculator on an ipad): it is about 1p from 50k a year. I calculated with 500 million not billion.