Earlier quoted context omitted.
In theory the children are committing a crime yes, but obviously enforcement is extremely low; left mainly to their teachers. I don't think UK law governs foreign companies' overseas operations based on the nationality of the customer though, no.
They’re not breaking any law. Laws apply to actions in the country, they’re not based on citizenship. If you go to Amsterdam and sleep with a hooker, you didn’t break a law by doing that: despite prostitution (specifically purchasing sex) being illegal in many western countries.
For example, Finland claims jurisdiction over crimes where the action itself or its relevant consequences happen in Finland or the victim is a Finnish citizen, permanent resident, or legal entity. Then there are plenty of rules and exceptions detailing what those principles mean in practice.