"Three Billboards" follows Andrea Arnold's "American Honey" (2016) and Edgar Wright's "Baby Driver" (2017) as part of a trend towards British auteur filmmakers writing and directing movies set in America.

But whilst the two earlier films were humanistic and optimistic portrayals of American life which didn't reduce Americans to stereotypes, "Three Billboards" feels trite and reductionist, and does not show the hallmarks of being written by someone who has ever spent much time in small-town America.

Apart from the strong accents there is very little attempt to create a sense of place - we get no establishing shots of the main street in the town or of its residential neighbourhoods, only individual buildings. We get a trio of memorable characters – the stoical and obstinate Mildred, the rough diamond Chief Willoughby, and the ball of impotent rage that is Jason Dixon. But some aspects of the character development feel perfunctory and even lazy, in that Mildred, (like most of the supporting characters), is defined by her relationship to the billboards controversy and very little else. For instance, it struck me as odd that Mildred appears to have no outside interests or female friends her own age, and I'm not altogether clear about how she earns a living - she works in some kind of touristy gift ship but it's unclear whether she owns the business or is just employed there. Peter Dinklage is a very good actor but he is given very little to do. James, the character he plays, is rather sad and pathetic and his entire contribution to the story, such as it is, is defined by his dwarfism.

In so far as it's a socially-aware film, the treatment of the political issues is muddled and self-contradictory. For example, early on it feebly tries to make a point about police racism, then later it shows they're not racist after all because they can accept a black senior officer's authority without difficulty...Meanwhile the two named black civilian characters don't advance beyond predictable behaviour, in that they are both anti-police.

This sort of cynicism and borderline misanthropy may be forgivable if a movie works on the level of pure comedy, but for me - and I am perhaps in a minority here - it simply didn't. I felt the humour was vulgar and unimaginative in the extreme and was overly reliant on violence, anger and profanity. Most of the audience at the sell-out UK premiere laughed audibly throughout - I chuckled maybe four times.

There were all kinds of little things that irritated me. Why does Mildred's daughter have Nirvana posters on her wall if it's set in the present day? Why does Mildred have to deliver that rant about pederasty in the Roman Catholic Church, when it has no connection to the rest of the story and when Missouri is mainly Protestant anyway? And do we really need to be told that she was victim of spousal violence in her past marriage, when the film has nothing original or meaningful to say on the subject? What's the point in creating a strong, non-stereotypical female character only to make her a victim of male violence twice over?

So, how can I justify five stars? Well, it's an unconventional story with unconventional characters. It subverts the familiar trope of the plucky loner fighting against overwhelming odds, and challenges the idea that protest, non-conformity and anti-authoritarianism are good things in and of themselves. Mildred behaves appallingly but has a completely unshakeable sense of her own righteousness. Perhaps a bit like some of the people who take part in BLM and Antifa protests and use them as an opportunity to start fights and create mayhem. The ending is pleasingly ambiguous; it leaves open the possibility of some kind of redemption for the characters, but at the same time it recognises that Damascene conversions are rare in real life and that many of us only learn from our mistakes as fast as we make new ones.

Go to see it expecting a sort of sub-Tarantino celebration of stylised violence and amorality and you won't be disappointed. As film critic James Verniere once wrote in summing up another movie, it's good nasty dirty fun. But the sad thing is it could have been so much more.