Every day at 11AM there is the changing of the guard at Buckingham palace. British guards march into the courtyard of the palace and arch back out. i think there is also a band.
this is a great place to get some good shots. Even though its a daily event its very popular and lots of people turn up. mirageii has some images of this above.
That's a great idea. I didn't think of that. Definitely worth going to see!
Last time I was outside the SIS headquarters (a few years ago), there were no armed guards (although they do have high perimeter fences) and taking photos wasn't a problem, but that was before the July 7th bombings in London, so security might be tighter now. The building was attacked by the IRA once. You might be interested to know that the government lifted the ban on photographing the building for the James Bond film (can't remember which one) which featured it, so now anyone can take photos of it if they want.