our responses to a film like this often reflects our own experiences. Those us who have been Emma Thompsons character see the film in a far more cynical way than those of us who haven't. Its quite telling.
I don't think that's true. I'm happily married to my undergraduate boyfriend, whom I've been with for almost a quarter of a century, and can see that Emma Thompson and Laura Linney act everyone else off the screen, and that it's not accidental that they're the figures of female self-sacrifice in the film, rather than the hot young things who inexplicably immigrate to shag random Brits, marry men they've never had a conversation with, or deal with the conundrum of having Andrew Lincoln and Chiewetel Ejiofor vying for your affections - and who aren't really characterised at all. (Why does Aurelia want to marry a slightly pudding-faced man she's never spoken to? Why are American women depicted as nymphomaniacs when confronted with a British accent, apart from Laura Linney? Would people who usually do porn body double work not be required to be more buff and tanned? Why is Laura Linney wearing a bag lady woolly hat at a church wedding?) 
Also, because several people have raised it, I think ET pretty much has to be plummy and Hampstead-y, purely because her brother is the PM, and no one casting Hugh Grant was going to have the PM be an ex-miner Labour politician... It makes more sense if they're a well-heeled, well-connected, posh-ish north London family.
Plus I actually quite like the bit where she encounters Hugh Grant backstage at the school play and goes into full on plummy luvvie mode - her character is in obvious pain but putting on a good face in front of the children and the Downing St 'staff' by being all annoyingly jolly. And of course it's poignant that what nearly makes her break down is that she believes that for once her brother has shown up to support her and see his nephew and niece, whereas he's just there for a shag with someone whose tendency to chirp 'Where the fuck is my fucking coat?' in front of a bunch of small children is supposed to be cute and shock the US audience... 