The agreement is one between two faded imperial powers, neither of which is able to quite let go of their own conception of their past influence and rôle, despite the fact that neither can really afford to compete in that arena, even if it made any sense for them to do so. They both wish to maintain the pretence of being 'world powers', although the world which they wish to inhabit is one which disappeared decades ago.
Their so-called 'independent' deterrents are completely useless against any of the real threats facing the world in the 21st century, and aircraft carriers are of use only in an offensive operation - even if they could afford to put any aircraft on them. So they continue to spend beyond their means on next to useless military hardware, whilst trying to save face and money through a limited pooling of resources.
It really is time for a more modern and realistic appraisal of the role in the world which the countries of these islands could and should play; but, not for the first time, the UK government has, instead, looked for a way of further delaying the inevitable.