I actually found this on wikipedia https://simple.wikipedia.org/wiki/List_of_countries_by_popul...:

"England officially became the second most crowded major country in the EU in 2008, according to figures released by the British Office for National Statistics"

I believe, it's because people from continental Europe sometimes don't differ between UK and England - but it's sensitive issue in the UK :-). UK is actually the first if we take European countries with over 20M people - 255people/km2.