Intelligibility of UK dialog?

Excuse me, but neither of you two know what you're talking about. The 'American' accent is descended almost directly to the modern era from the Tudor-Stuart accent prevalent in the late 16th- to early 17th C. era when the first two of the Original 13 Colonies were founded. The language, isolated as it was, from the mother tongue, became 'frozen' in time. It gifts modern English speakers with the closest window they will ever have into the sound of the language as it was spoken by the likes of Elizabeth I, Shakespeare, Ben Jonson, Francis Bacon, et. al.
The most significant modification to this paradigm came from the huge Scots-Irish influx into the trans-Appalachian--Ohio Valley or 'old Midwest'-- region between the time of the Jacobite Rebellions to the Irish Potato Famine era (roughly 1700-1850) and which coincided with American westward expansion. Thus, the so-called 'Midwest' American Accent, which has the flattest and most discrete intonation of vowel sounds, is the modern descendent of that English/Scots-Irish blending. If one listens to Irish English, one can immediately hear from whence Midwest Americans get their vowel pronunciation (and to some degree, their syllable stress and cadence).
Not surprisingly, then, Irish actors prove to have the best Midwest American accents onscreen; and, the crème de la crème of these is, in my opinion, Brendan Gleeson. His is flawless and very natural. One would NEVER know he wasn't a native born American. Close and phenomenal runners-up from amongst English Brits would be Bob Hoskins and Jason Isaacs. Granted, these two have not produced a pure Midwest accent, but rather, the 'Brooklyn/Bronx' and the 'Boston' working-class accents, respectively. Of those able to produce an oexcellent 'Generic Southern' accent is the current star of The Walking Dead, Andrew Lincoln. Of costar David Morrissey, not so much. English actors tend to 'get away with' a semblance of an American accent if they're able to disguise their vowel pronunciations in various regional dialects.
British English began its drift away from Canadian/American English around lthe 1730s under the rule of the Hanoverian German Georges, whose earliest monarchs never cared enough about their English subjects to learn the language properly. Indeed, George I never bothered to learn English at all, nor even to go occupy his new English throne, preferring instead, to remain in his cold and dreary northern Germany. Go figure. The second George was little better. He ferried back and forth between Hanover and London and spoke English with a heavy 'Cherman' accent his entire life. While George 3 was the first to be born and raised in England, he and his family spoke German at home amongst themselves. Thus the 'King's English' began to become bastardised with European vowel pronunciations introduced, ironically, by the royal family, itself!
The 'Midwest Accent' then (which our Canadian brethren share with us) is the standard one which is taught to everyone entering the broadcasting trade and to those foreign students wishing to learn 'American' English. And for your information, no one I've ever spoken to pronounces 'calm' with an R. Even in Boston, the word is pronounced 'caahm' which a broad A sound close to the A in apple, and with a lightly aspirated H.

"I say we take off and nuke the entire site from orbit...it's the only way to be sure..."

This is why phonetic guides were introduced. Think of it as calm as in karma, and your version as calm as in cam(shaft).

Does that help?