As a fellow photographer, believe me, it is perfectly legal to take pictures of anyone or anything provided both you as the photographer and the thing/person are in a public place, or the thing/person in on view from a public place without the aid of hoists, high ladders or telescopes/mirror contraptions. The safest rule is if you can walk down the street (or other public place) and view the person/thing normally there is no legal restriction to photographing it.
There can be no objection on grounds of privacy, even if the person is in a vehicle, as it is a PUBLIC place.
There could be other restrictions on you personally or your subject that would bring you into conflict with the law or the courts. You must not encourage or perpetrate an illegal act or breach of the peace in order to make an intersting photo. You must observe any injunctions that apply to you, your crew, or actors. For example a judge may have prescribed a curfew or a restriction to movement for a person; for example during a suite for harrassment or persistent petty offending.
Remeber, not all places to which the public have lawfull access are deemed Public Places. Most shopping malls (Blue Water, Brent Cross, Metrocentre, Eldon Sqaure Milton Keynes Centre Blackpool Pleasure Beach for some) are private places and permission is usually forthcoming if you apply to the Management Suite or Publicity Office in advance with details of what you want and in what way the eventual material will be used.