LONDON • The noisy demonstrators outside the British Parliament buildings in the heart of London have disappeared. And so has the excitement generated by the novelty of the occasion; the first week since Britain left the European Union passed uneventfully.

However, a great deal has actually changed, even if little of it is presently visible. For the first time in half a century, the British are called upon to redefine their place in the world, pick the policies they wish to pursue and pinpoint the dangers they wish to confront. The task is just as momentous as that which faced Britain at the end of World War II.