European Physical Society which produces this journal is unrelated to EU, national physical societies have its membership directly without involvement of their countries' governments. IOP Publishing which publishes this journal is based in UK. "IOP" in "IOP Publishing" is Institute of Physics which is also based in UK.

Equating being "a European nation" with EU membership is surely a trolling.

We will most certainly still be a European nation. Just not a member of the European Union. We are still located on the European continent. Its spelled centre.

“Some years ago I opined that London was not really an English city any more. Since then, virtually all my friends from abroad have confirmed my observation So there must be some truth in it.” -- John Cleese

In that sense, perhaps London remains a European city while still the capital of a not really European nation.

The UK will no longer be a European Union country but it will definitely keep being European. And I guess Ireland has a "centre" too.

Centre is the standard for all English countries except the US, no? We use centre in Canada

This was the work of spelling reformer Noah Webster[1]. He was quite influential in the United States, but unsurprisingly the British Empire was less enthused by his early 19th century linguistic innovations.

[1] https://en.wikipedia.org/wiki/Noah_Webster