“Ancient lights” is a colloquialism for the “right to light,” guaranteed under English law, whereby windows that have seen twenty years’ worth of “uninterrupted” daylight cannot be blocked by the construction of new buildings…
“[i]n the center of London, near Chinatown and Covent Garden, particularly in back alleyways, signs saying ‘Ancient Lights’ can be seen marking individual windows.”
So say Bldblg and Wikipedia