On the whole, UK law doesn’t prevent photography in public places. In the United Kingdom there are no laws forbidding photography of private property from a public place. Photography on private land is not restricted if the landowner has given permission. However, landowners are permitted to impose any conditions they wish upon entry to a property, including forbidding or restricting photography. Source wikipedia
Practical photography explains
Photography in Public Place
In the UK you do not have to get the permission from people you photograph whilst they are in a public place. Using and selling images of people in a public place is usually acceptable if undertaken with a view to being used for any journalistic or artistic material.
However if you intend to sell the image commercially or use it for a commercial purpose (for example to promote a product) it is normally recommended to get people to sign a model release form. Click here for further reading on BLPA Awards