For the sake of accuracy the Office for National statistics has estimates and projections for the years in question of around 725k reaching 18. I am beginning to think all the figures are inaccurate apart from those supplied by the Electoral Commission. Decide for yourself.
https://www.electoralcommission.org.uk/find-information-by-subject/elections-and-referendums/past-elections-and-referendums/elec