This magnificent butterfly is generally not common, though it is widely distributed throughout Europe, extending by migration to places where it is not resident.
The butterflies emerge in high summer and disperse, when they can be found pretty much anywhere, though they are most likely to be encountered in forested regions. They then go into hibernation at sites where the foodplant (principally Salix alba but also other willows or poplars) is plentiful. Such sites may be used year after year and it is easier to find this butterfly in the spring, by looking in known hibernation sites, than in the summer. The flight period is also longer in the spring, in my experience.
The adults take the nectar of many plants but also enjoy minerals on the ground. Here is a video of the first Camberwell beauty pictured above doing this, and sunning itself at the same time:
