I was under the impression they were there to protect a nation's security against foreign threats, not to spy on delegates at international conferences for economic and political advantage.

If our behaviour is essentially amoral and we are willing to break diplomatic treaties and international law in secret (as you are advocating), we should expect the same in return, and have no right to talk about international law, human rights, democracy, national sovereignty or expect anyone else to respect them when we need them. Why shouldn't Russia simply cut off the gas supply to the UK next winter when they don't get what they want, or start murdering government ministers rather than dissidents with Polonium? Realpolitik is a dangerous game to play, particularly when you are a declining power (as the UK is), and I think it better we at least try to stand up for the values we think are right, and hold ourselves to the same standards we would hold others to.