Earlier quoted context omitted.
You're right. Mr Cook has good reason to P45 (British pink slip) the UK legal team for their poor advice, but a fish rots from the head.
But gangrene frequently afflicts extremities. And fish heads are not often replaceable. Metaphors often (generally?) make poor arguments for why X does Y.
The early bird gets the worm, but the second mouse gets the cheese, and all that.