The actual statistic is about people who identify with the ethnicity category "White British". And indeed that group might well be a minority by that date, if you were to treat the current set of people as the baseline and the categories as immovable.
But remember that there are 5 million EU citizens in the UK today. They are overwhelmingly White, although they are not White British. But if they stay in the UK, their children and grandchildren will be White British, just as your friend with a Polish surname whose grandpa fought in the Battle of Britain is White British.
And another chunk of the population are Black and mixed-race people of Caribbean heritage, often third-generation. They may not have perfectly white skin but they are as British as anyone else, unless you are some kind of blood-and-soil absolutist. (And of course many of those would be in for a surprise or two if they were to trace all of their ancestors.)
And so on. People become British by growing up in the UK, and they both absorb and slightly reshape the culture, which has evolved. (I can remember when being horrible to gay people was part of the culture, not much more than one generation ago.) Sadly they will not become more white, so if the literal colour of people's skin is an issue for you, you're going to have to deal with it, or perhaps move to Greenland. (But when the US annexes it, a lot of the soldiers will be Black. Imagine that.)
So this is just another race-baiting statistic designed to get people angry. I wonder who has an interest in doing that? Surely not OP, who is doubtless just asking sincere questions.