I suspect the easiest way would be to ban advertising things designed to appeal to children, which is content-based rather than reader-based and cuts with the grain of some previous restrictions in the UK (no junk food ads on children's TV, for example, no tobacco add at all). It's intent-based and so less gamable, and would tank the value proposition of British kids online, if enforced, by lowering the CPM rate.
I'm not convinced it's a good idea, FWIW, but it would be a lot less crappy than age verification.
I'm not convinced it's a good idea, FWIW, but it would be a lot less crappy than age verification.