you still haven't found one that stops you doing what you want to.
If you form a community, and agree that the members of the community won't, for example, throw sewage into the sea and rivers, for the benefit of all members of that community, that agreement is ineffective if any of the members of that community can throw sewage into the sea and rivers. So to have any effect, you, as a member of the community, agree that the community will set up a mechanism to monitor and regulate its members.
As you say, Britain might otherwise throw sewage into the sea and rivers despite promising not to.
Which it does.
And it might declare that the sewage companies aren't obliged to monitor and report on their sewage-dumping
Which it does
And it might fail to uphold the rules it agreed to
Which it does.
And you, as a British subject, may find you are bathing and fishing in sewage, and you might be powerless to make the government fulfill its obligations.
Which you are, except that you currently have the right to appeal to the communal mechanism which Britain committed to respect. And which is less ineffectual than you are.
Grow up.