I fail to see how this protects children from those evil people, tho.

It establishes the age and id of UK children to websites and services. However, unless all these services children use are siloed off from the rest of the internet and UK only, bad people from other countries (and those in the UK savvy enough to mask themselves behind some kind of VPN/TOR) will still be able to use these services without having their ids established the same way, and will keep trying and sometimes succeed to groom and abuse children.