At first I struggled to answer that question ,perhaps because of the diverse genre of modern movies. To attempt to answer the question I began wondering traditionally what it was that made us Brits... Brits and I think it has to be the Heritage of our armed forces.
I think the legendary British 'stiff upper lip' came from the exceptional discipline instilled in our Army and Navy .
That discipline together with all the best virtues like honour, loyalty, courage, valour, steadfastness,fortitude etc filtered down into general society and into the family unit where once upon a time these virtues became something everyone admired and aspired to from an early age and was how our movie industry traditionally depicted us Brits.
I think up until this Millennium the movie industry has been responsible for depicting Americans as Romantic,brave,brash, forward, gung-ho, etc and the fact that in the past 50 years most war movies depict the USA as having won WWll single- handledly with the Brits just tagging along for the ride didn't exactly endear the Yanks to the Limeys of my generation ,especially as you always seemed to have too much of everything and we endured crippling austerity and rationing up until 1955.
International travel in the last 30-40 years and a more balanced media presentation has long since eradicated that view and frankly most of that generation are now dead!
I would say in answer to your question I think modern movies depict the Americans as they are , a warm, friendly , good hearted ,generous, industrious and innovative nation prepared to stand up and be counted where there is injustice or oppression.