You don't need any sort of permission or agreement to just take a picture - especially from a public place. The only time a release will actually be needed is if there is some sort of dispute and you need to prove that you have permission to use the image in a particular way.
There is no legal requirement as such.
However, some stock agencies/image libraries will cover themselves by insisting upon a model release for any recognisable person in an image.
Some UK relevant guidance here: