in what year did the population of the English reach 1.5 million people?

Answer 1

Intriguing question worth a star.
The estimate for 1066 CE is 1.1 million
and for 1215 CE 2.5 million, so I would hazard a guess
that it crossed the 1.5 million mark in the 1100s CE.
That was an interesting time period in English history.
There was civil war between Empress Matilda (or Maud)
and King Stephen between 1135–1154, so I'd bet England reached 1.5 million during that time or certainly by 1154
when Henry II became king. Henry was the first Plantagenet
and the father of Richard I 'the Lionheart'.
There is a sticking point there however. Some - a minority - of the population of England did not speak English. They were Norman French. Maud, Steven, Henry II, and Richard the Lionheart would have spoken French preferable to English if they spoke English at all. So it depends on what you consider the "English" population.
Still, it was probably sometime in the 12th century.