What you're seeing is population momentum.
Women in their twenties and thirties are having far fewer than two babies each. In the long term that will lead to population decline (if you ignore immigration).
But there are far more women in their twenties/thirties than there were in their grandmothers' generation, because their grandmothers had almost three children each on average (and almost of them survived, because they had access to fairly modern healthcare). And those grandmothers are the generation who are mostly dying at the moment and being replaced by their granddaughters babies.
A fertility rate of 1.61, which was the England and Wales calculated figure for 2021, will lead to significant population reduction, but not for a few years yet.
However it shows up in primary schools much quicker - a lot of people assume that their schools are full because that's what the local grapevine has always said, but a lot of them actually aren't. It depends on your area, but if you check the records then loads of formerly oversubscribed primaries are now open to anyone who wants a space. In inner London primary schools, and some secondaries, are being closed.