In hackney (East London)you are not allowed to stand on the grass for the reason below:

https://www.atlasobscura.com/articles/londons-plague-pits-map-shows-where-the-black-death-got-buried

They do not have the same signs at Green Park and Shepherd's Bush Green, that also contain plague pits. Even Sainsbury's in Whitechapel is built over one.