Downing Street

Set expectations first: you will not walk up to the famous black door. The view is through the tall black gates on Whitehall, and the closest you get is the sidewalk outside them, so a photo and a few minutes is the realistic plan. The gates are themselves a lesson. Pedestrians could still walk to the door for photos after vehicle access was restricted in 1973, but railings went up in 1982 in response to IRA terrorism, and the ornate gates followed after planning approval in 1989. Number 10 is the prime minister's office as well as home, and the government departments lining the street around it are why "Whitehall" became shorthand for the civil service.