Taking Pictures....... - Do you ask permission?

To all experienced on the ground reporters and DJs: Do you need to ask permission? I happen to pass by the US embassy in London this morning and decided to take a picture of an armed body guard with a very sophisticated gun. I asked him if I could and he said " No" and turned his back on me. The US embassy is just surrounded with armed men and police cars. A sight to remind everyone of the military might of the US.