Photographers’ rights in the UK*: Basically, you can photograph anyone or anything from a public place. Period. 6’19” in on the video.
Caveats: Photographing Ministry of Defence (MoD) property might sometimes be in breach of The Official Secrets Act, by which everyone is bound whether or not they have signed it. In this era of increased terrorist threat tension, you might also arouse police suspicion if you’re taking detailed photos of known sensitive sites or if you’re repeatedly photographing an individual and it seems like harassment.
But, the police do not have the right to stop you photographing and certainly no office building door staff can, if they try to and threaten you or try to take your camera, you could have them arrested for assault and attempted robbery.
*This is under the law of England and Wales, I assume it is valid in Scotland and Northern Ireland too, but there may be variations on the theme there.