On chrome on windows desktop in North America, when I click the BBC link I get the paywalled link and then almost immediately a redirect happens that sends me to a rendering of the PDF article. Same thing happens on firefox on windows desktop.

The top right of the PDF-alike has a link that says "what's this". and it sends me to https://www.springernature.com/gp/researchers/sharedit which seems like a service that allows sharing of links to fulltext. If I go to the url directly, e.g. by copy-pasting the URL or clicking on your link, then I do not get the same behavior. Hence, I deduce that it is probably some referrer magic.

When I click the same BBC link on chrome on android mobile, I don't get the redirect, so probably there is some User Agent stuff too.