I am writing to let you (and a certain number of your population) know that, contrary to popular belief, not all British people speak the Queen's English.
While TV shows such as Downton Abbey do depict the British people (a small, wealthy minority of the British people, I might add) as adopting what is known linguistically as received pronunciation, the great majority of Brits don't, and never did, speak this way.
In fact, Great Britain is blessed with an extensive network of wildly contrasting accents - and indeed dialects - from the most southern tip of Cornwall to the Outer Hebrides in Scotland. Among constant repeats of various Hugh Grant films, I am not altogether surprised that Britain's accentual versatility is lost on many of your people.
Indeed, I have noted that many of these same people, from time to time, like to "impersonate" a British person by adopting the Queen's English—or some under-rehearsed interpretation of it. While I am truly in favour of sampling another country's brogue, it would not harm your countrymen to attempt West Country or Glaswegian from time to time.
When I think of British accents, I think of Scouse (regional to Liverpool), Mancunian (Manchester) and Geordie (Newcastle). I think of Brummie (Birmingham), Broad Yorkshire (Yorkshire, obviously) and Cardiffian (my way of saying Cardiff-speak).
In fact, I pretty much think of anything but the Queen's English.
Now I know what you're thinking: Brits are often pretty lousy at American accents too. Believe me, I aim to raise this matter with Britain in due course.
But I must conclude this by saying that, and I'm mainly talking to 8th grade performing arts students here, raising your intonation at the end of a statement does not make you sound British. In any way. So stop.
A British expat
P.S. Dick van Dyke is not a good role model when learning Cockney.