My belief is that, even in the UK, it might be more likely for Charles to be pluralised Charleses. The Grauniad (in many other ways one of the world's best English language papers), can sometimes demonstrate a cavlier attitude to the language. The number of times its writers 'beg the question' inappropriately (at least for this logic-dillettante), is past count.

To be honest, I'm quite comfortable with the idea of using an apostrophe to account for the plural in such a case, but that may be because I enjoy using apostrophes wherever I can. In any case, I suspect that even the writer in The Guardian would expect the word to have been pronounced (in the reader's mind) as Charleses.

cheer

the sunshine warrior