Astronomical twilight does not end on nights near the summer solstice in Southampton, England.

No, astronomical twilight is the darkest stage of twilight where the Sun is between 12 and 18 degrees below the horizon. Near the June solstice in Munich, there may be some residual light in the sky even during astronomical twilight due to the high latitude of the location. The sky may not get significantly darker than astronomical twilight on those dates.

No because the latitude is 55 degrees N so the Sun never goes more than 11½ degrees below the northern horizon, so nautical twilight and astronomical twilight do not end.

When the Sun is 18 degrees below the horizon.

Yes, absolutely. At the Arctic Circle in December, it barely gets LIGHT enough to be astronomical twilight. Then it gets dark again.