I understand that in practice driving test centres tend to have their own temporary rear view mirrors for examiners to use. I understand the page at https://www.gov.uk/practical-driving-... reflects current government policy of putting the responsibility of providing a mirror on the applicant however it might be useful if it reflected the reality which might prompt applicants to check if their test centre had a mirror which could be used.
It might be that even drawing attention to this issue in the right place could prompt a policy change and get examiners who'd use the mirrors every working day to provide them, rather than applicants who use them once in a lifetime. (I realise this isn't the place to push for policy changes!).