"Whoever" Vs. "Whomever"

On the subject of "whoever" and "whomever", I was reading this but I am still confused: http://www.grammarbook.com/grammar/whoever.asp

What is the correct use of whoever/whomever in the following sentence?

I like your copy, congratulations, whoever is writing it.

Whoever? Whomever?

Is there a difference between US and UK English?

The answer is that it has to be whoever, because the relative pronoun takes the case of the function it serves in the subordinate clause. That whole clause is “whoever is writing it”, where whoever is the subject of the clause, just as is is its verb.

Swap in he-vs-him on things like this to see which one works right: you would never say **him is writing it*, so it cannot be whomever.

No, this is not different in any standard form of English, although some hypercorrectionists are wont to get it wrong and wrongly stick whomever in there where it has no business being.