It's clearly incorrect to say that 52% of the UK population voted to leave the EU in the 2016 referendum. The estimated population of the UK was 66,040,229 in 2017. The 17,410,742 people who voted to leave the EU represent almost 52% of the people who voted, but only a little over 26% of the population.
Having said that, I think we all knew what the question setter meant.
Of the 46,500,001 people registered to vote, 33,577,342 actually voted. The turnout was just over 72% – not bad by normal general election standards, particularly in recent years.
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