That's right. The Crown Prosecution Service (the national body which prosecutes essentially all criminal cases on behalf of the state) takes the view that charging under-18s for genuinely consensual sexual activity is generally not in the public interest. This policy (and guidance on essentially all other charging decisions) is helpfully published online as part of the CPS legal manual.

It's still a matter of prosecutorial discretion rather than being actually legal, though.

What does that have to do with sexting?

Because it's a sexual activity, illegal if done with pictures of under 18s (but not generally considered in the public interest to prosecute by the CPS if the pics are of the perpetrators themselves and it was consensual - so no "revenge porn" setting or whatever).