I very much doubt if you could detect any natural scattered sunlight in the sky after the end of astro twilight from anywhere in southern England. I’m not sure how stable the SQM photometer is but I imagine you are detecting man-made scattered light. You would need to have a very good sky for natural sources such as airglow to have a detectable effect.
I attach a plot from “Sunsets, twilights and evening skies” by Aden and Meinel. This is a lovely book and has several chapters on twilight. Also, a plot from an imaging session on La Palma where the skies are really dark and, in this case, very transparent . This shows the median sky background for several imaging sessions of objects at different altitudes. Astro twilight begins at 04:52 and it is only just detectable even from a place with very dark skies.
It’s all a bit academic for me in Chelmsford. On very good nights I can only just detect the sun in my skies when it is between 15-16 deg below the horizon!
