The reason teeth can grow crooked is because modern food is simply physically easier to eat now than it used to be and we therefore don't spend all day chewing on tough foods, strengthening our jaws, and keeping our teeth aligned. It's not a Brit thing, or an anyone thing. It's evolution, baby.
It's also fine to have teeth that aren't perfectly white and straight. This guy is not only saying British Asians aren't British, he's also saying that to not meet incredibly narrow beauty standards is one of the worst things that can happen.
Yup! I figured I'd zero in on this specific item of bullshit and let more qualified and informed folks handle the larger argument(s) that I'm very much here for but are less in my lane.