I found a solution to give Affinity Photo access to my Photos library. On the Affinity Photo startup screen, I long pressed on the downward pointing triangle that is part of the New icon. I was presented with a list of options by which I could create a new document. One of these was "Import from Photos". When I selected that option, I could give Affinity Photo access to my Photos library. I was then allowed to select an image from the Photos library to import into Affinity Photo and start editing.
Open still does not list my Photos library as an option, but I can import images from my Photos library by long pressing on the New icon. See the attached screen shot.