In some countries (including the US) you can additionally "register copyright", and it makes enforcement easier or allows higher compensation, maybe that's what they're referring to.

That advice wouldn't help the british creator in this case though.

Correct, in the US there is implicit copyright by default if you include a copyright mark and date and formal copyright through registration. Implicit US copyright doesn't offer much protection, just an ability to tell people to stop using your works. Formal US copyright gives you rapid retort and default judement compensation that's just insane (something like up to $150,000 per infringement). You don't have to be a US citizen to register for a US copyright.