I'd agree it's essentially just a diplomatic statement, even if Cameron probably felt it to be a bit obtuse. When it's your closest and most important ally speaking, you have to listen, especially when that ally is much stronger than yourself in many ways.
People around the world are used to the U.S. sometimes giving advice or making assessments in public about what other countries ought to do, or need to do. So it's sort of a funny sidelight when other major countries do the same thing at the U.S. - I remember how some Chinese bigwig, if it was Wen Jiabao or somebody close to him, spoke up during one of the rounds of American senate budget talks last year, and the gist of what he said was (paraphrase) "You need to get your act together on Capitol Hill - we the world can't have it like this, can we?" I'm sure the Chinese (and many other people around Asia, or in Latin America) loved hearing the Peking top brass making that kind of punchy advice talk in return after people in those countries had been looking up into the sole of a western - sometimes U.S. - boot at many times in the past.