Why does Atonement knock Britain?

Watched “Atonement”, starring Keira Knightley and James McAvoy, the film version of Ian McEwan’s novel and the latest British costume drama which American critics affect to love (hence talk of Oscars) but to which American audiences are indifferent (in two weeks in the US it’s taken only a paltry $3.5m at the box office).
 
Atonement is fine as these things go, yet another depiction of class-ridden Britain in the 1930s, which encourages global audiences to think we’re still the same out-dated stratified museum in 2007.
 
In general the film depends on high-quality dialogue and lovingly-depicted country-house scenes, which are cheap to shoot. Most of the budget, however, seems to have been blown on recreating the evacuation of Dunkirk in 1940. This was shot in Redcar (honest) and has a Hollywood scale to it, quite out of kilter with the rest of the film.
 
Just why so much was made of Dunkirk is something of a mystery because the scale and scope of these scenes were hardly necessary to the story.