Full astronomical darkness is now returning to the southern UK. Here's a chart from FLO's Clear Outside showing annual darkness for Bodmin Moor.
In the blue portion of the chart, centered around Midsummer, the sky never becomes fully dark. At that time of year the Sun never drops very far below the horizon, so the upper layers of the atmosphere are still lit up in an extended twilight. This isn't easily noticeable from brightly-lit towns and cities but is quite obvious from a dark, rural location. Astronomical images taken outside of astro-dark have a blue tint and are washed out to some degree by the reflected sunlight.
The 15 second exposure above was taken on the 18th June in SE Herts, at about 11:30 PM. The yellow colour at the bottom right is caused by light pollution, probably from the town of Bishop Stortford.