UK laws allow abortion at any time for fetal anomaly—even if the anomaly is not life-threatening or painful. Something as minor as a cleft lip can be a death sentence for an unborn human in the UK.
There are many possible ways to be intersex. This includes but is not limited to mutations on the X and Y chromosomes. Unborn humans with these mutations detected are frequently killed for chromosomal abnormality. Prenatal testing companies already “screen out” intersex humans with these tests: https://www.preventiongenetics.com/testInfo?val=Disorders-of-Sex-Development-%28DSD%29-Panel
Structural abnormalities may also be detected by ultrasound.
Natural rate of occurrance for intersex people: “An estimated 98-99 percent of humans are either blond or brunette. Despite this, we have accepted—rather than denying, hiding, or attempting to eliminate—the existence of redheads, who, interestingly, make up the same percentage of the population as intersex people (1-2 percent).” — Hida Viloria and Dr. Maria Nieto in The Spectrum of Sex: The Science of Male, Female, and Intersex (page 24) https://www.amazon.com/dp/B07ZPH6H4T/ref=sr_1_1?dchild=1&keywords=the+sex+spectrum&qid=1588641521&sr=8-1
Intersex is not a gender. Since you mentionned intersex people, I'm going to talk about biological sex here, not gender. There's a whole discussion about whether or not those terms mean the same thing, but for the purpose of my message I'm writing about biology research and knowledge, not social studies. But my last paragraph also answers the question of gender and intersex.
So, first, interesex is not a sex either. It's an umbrella term for everything that doesn't fit in the two pretty narrow boxes that are "male" and "female". There are millions (maybe tens of millions, scientists argue on the exact number) of people that, at birth, does not fit in those boxes. The scientific consensus has evolved away from the binary notion of male and female, and towards the idea that sex is a spectrum.
Science does not backup "there's only 2 sex" anymore; nor "3 genders", for that matter. We hold on to this vision because it's simple, because it's what we've been taught as kids, because it's an idea promoted by figure of authorities we respect, because our society and education system lags a lot behind scientific discovery (even med school tends to lack behind actual biology research on multiple topic, making physician discourse and practice evolve slowly), etc. We hold on to that idea because it's easy and comfy and we don't like having our preconceived notions shattered. But yeah, Santa doesn't exist, Jesus didn't hang with dinosaurs, and biology is more complicated than a shape sorter toy for kids.
There are plenty of things you can read on the matter, a quick google search got me: a well-sourced short text on intersex people, a book from an actual professor of biology specialized in sexual diversity who also happens to be intersex, an article by a science journalist who interviewed multiple searchers in biology to try and find an interesting way to represent what "sex is a spectrum" actually means and another sourced article written by a biologist researcher, for good measure. Surely enough, you can also find a rebuttal for these articles, coming from an evolutionnary biologist.
So if some researchers say it's a spectrum and some say it isn't, what to make of it? Well, you decide for yourself, and you can most certainly decide that "there are 3 sex". But considering people who've spent years, sometimes decades, learning about this subject in-depth can still not agree on this but that everyone at least agrees that the two boxes system have failures, maybe people on the internet should not be so certain of their beliefs, especially the ones that were taught to them at an age when they couldn't even wipe their butt. Biology is a mess.
One thing for sure, though, the idea that "people must fit into those boxes" lead to the mutilation of millions of babies around the world and that's just not ok. And honestly I don't think the "3 sex" stance help at all to that regard. As long as intersex people will be seen as "other", physicians will push for unnecessary surgery that can have life-lasting consequences just to push someone into an arbitrary box. So while you can very much have the beliefs you want, pushing for this one has very concrete repercussions on the lives of millions of people, usually creating trauma (both physical AND emotional) during their childhood. We can do better than that.