The crash is 15 seconds into this video:
What happened was more mundane than a Delta barrier going rogue, but every bit as embarrassing. The Presidential limo is so long it couldn't clear the slope of a ramp that connects the embassy to the public street. That incredibly super-secure, multimillion dollar, product of Yankee ingenuity high-centered on a little ramp and was stuck there until it could be lifted off.
Doesn't the Secret Service check out the routes the President's motorcade will take? Of course it does. I found a UK news article about Secret Service advance planning for a previous Presidential visit to London which refers to a 500-item checklist:
As the manual states, ‘Advancing is an art’. In eight years, a US president makes about 3,000 public appearances, 800 of which will be abroad. Each foreign appearance requires a site survey by more than 100 Advance Team members, more if it’s a RON (Remain Overnight Visit).
The checklist for the UK Advance Team contains almost 500 items and stipulations covering 25 pages.
These include the effectiveness of the motorcade in rush-hour traffic, how to address the Queen, approval of presidential hand-shakers, no animals, no children (if possible), certainly no parachutists or balloons; then there are sketches, photos, 3D graphics of Heathrow, the Ambassador’s residence and grounds in Regent’s Park, departure points, corridors and walkways.
"Advancing is an art." Maybe that's the problem. It should be a science, too. Motorcade operations are so important to the Secret Service that I take it for granted they must be the masters of route reconnaissance, and expert judges of all kinds of vehicle dynamics like speed, acceleration, braking distance, turning radius, side slope stability, and so on.
But, evidently, the motorcade checklist does not include anything about angles of approach and departure. Some advance agent's career probably bottomed out along with the limo.