The below excerpt is taken from the Metropolitan Police website http://content.met.police.uk/Site/photographyadvice
…….Freedom to photograph and film Members of the public and the media do not need a permit to film or photograph in public places and police have no power to stop them filming or photographing incidents or police personnel………..
You have the right to film anything or anyone in a public place, including the Police. The safest you will ever be whilst dealing with public servants or debt collectors is with a camera in your hand. Nearly all of us own a phone with a camera so it is now easier than ever.
Once they know you are filming their attitude, most of the time, completely changes. They are less likely to trample over your rights and you have evidence of the whole instance.
Remember, no one has the right to stop you filming in public or on your own private land. If they tell you “It is against the law!”, simply ask them “what Law?”
Chief Constable Andrew Trotter’s (Chair of ACPO) guidance letter to Police ref: filming and photography in public.