I would re-emphasize what FFF says - if you notice a 'standard passport photo' is not square wheras a US Visa one is...
I was with someone who had a UK passport photo accepted while I was there, but I really wouldn't count on it, that was a few months ago and they have got even more anal since (if that was possible). I beleive there is a 'Snappy Snaps' a few streets away from the embassy if you haven't got one you can get to before you leave (the list assumes everyone in the country lives in London if I remember rightly).
Take a good Book and be prepared to practically strip in the street to get through the metal detector on the way in
