A photo pass thing**
Bonus points if you can demonstrate how such a system would comply with domestic equality and data protection laws and international human rights law.
Having to wear it round your neck probably wouldn't. Simply owning a photocard wouldn't break any law. In fact it IS the law in other places.
A lot of countries have photo-ID cards as identification. In Spain they're compulsory to have from the age of 14, and compulsory to carry around with you everywhere from the age of 16 (I might be wrong on this one). The police have the right to ask you for ID anywhere.
A few years ago they introduced the e-version which everyone now gets when you're up for renewal. The chip contains my fingerprints, more photos of myself, all my personal data and police records if I have any.
We also have health cards but without a photo. You get one at birth. All your medical history is stored on them. When you ask for a GP appointment they scan your card (or you give the long number out on the phone). Your prescriptions are on the card. When you go to the pharmacy they scan your card again. Similar healthcare system to the NHS so everyone has the same one.
It would be as simple as adding 'EXEMPT' to one of those. It doesn't have to state the reason and no one should ask why. It's working in other places so I don't see why it wouldn't work in the UK.