You should document consent every time you take a photo.
"[You] must not collect sensitive information about an individual unless: " … "the individual consents to the collection of the information"
"[You] should generally seek express consent from an individual before handling the individual's sensitive information".
"[You] should as far as practicable implement procedures and systems to obtain and record consent."
Doctors often don't get consent, and when they do, they don't record it properly. A whopping 82% of the time, doctors don't document consent when taking a photo.
A study among dermatologists revealed that only 2% obtained written consent! While 46% received verbal consent, they failed to document this. A further 30% acquired verbal consent and documented it in the clinical notes, but they did not record it using an appropriate consent form.
We suggest you always obtain consent. The ease of distribution of digital images, the difficulty of properly de-identifying photos, and the grey area around when consent is required, in practice, means consent should be sought to cover yourself. Being able to point to documented consent may save grief should dispute arise in the future.