Where are you getting that information? If you have a National Insurance number (and date of birth) both the paper form and digital route don't require any documents to be provided. The digital option requires a digital photo that has similar requirements to a passport photo (against plain background, head and shoulders, no filters etc), it does not need to be a passport photo. A high quality scan might be accepted, but it's expected that people send a photo taken from a phone/camera or send the digital version from a passport booth/photographer if they did that for something else.