To echo the Prime Minister, I would just read briefly from our commentary on this very point:
What Article 21 should allow in these situations, therefore, is for a State Party and the members of its armed forces to call in a strike delivered by a State not party in an international operation even if there is no guarantee that cluster munitions will not be used. Military personnel would thus not incur penal sanctions for the use of cluster munitions by others as long as they do not exercise effective control over the tactical decisions following their request for support.
So other states have considered this and haven't found it necessary to come up with the very broad exceptions that clause 11 has.
One of the difficult challenges that you as policy-makers and those who enshrine the laws of armed conflict have, is that even in those difficult situations, even in the fog of war, there's no excuse for violating the law of armed conflict or violating treaty obligations related to inhumane weapons.