carrying ID is not required in the UK. The police can ask for proof for identity and address under some circumstances (e.g. they need a reason more than just "

carrying ID is not required in the UK. The police can ask for proof for identity and address under some circumstances (e.g. they need a reason more than just "you were protesting" and I think generally this would take time and paperwork, including the police giving a receipt to the person. Much easier and cleaner to take a photo.