I've just found a photo of my Grandmother and a friend/sister(?) - it also is a Williams' photo. The inscription on the back is 'Williams' While You Wait Photo Post Card' and the address given is '82 Seven Sisters Road and branches'. Seven Sisters Road runs from Holloway in North London, east to South Tottenham. I suspect 82 was at the Holloway end, as that's where my family are from. Sadly, the photo is undated.
I'm actually just starting to search on this and your post was one of the first on Google!