It would need to be published in that country.
I am not sure what the argument on jurisdiction would be if it is only published as a consequence of the defamation (if that is what it is?) i.e. only being re-published as a consequence of the publicity regarding the defamation? Thus, can the victim (if that is what he is?) mention it, seek publicity for it, cause it then to be viewed millions of times on You-Tube, and then sue for those publications?
I'll leave that to the lawyers.
He would have less difficulty if he sued in London. Then again, would a sitting US President like the optics of wigged and gowned English lawyers determining the matter without TV presence?
He chose Florida as he has a home there, it is in the USA, and (I suspect) has generous libel laws.