This is one of the funniest films I've seen in the last few years. Basically, it's a parody of all the cop-themed 'buddy movies', especially the worst ones (it has plenty of references to Point Break and Bad Boys, for instance).
First of all, it's a British film, so it has penty of that British humour I love, and obviously the cops aren't as 'bad ass' as the ones depicted in American films: they eat chocolate cake, drink tea, don't carry any weapons — which makes the parody all the more irresistible. Second, the scenario is really, really good: you start thinking you're in for a funny but scenario-less film and slowly you realise there is a plot. And then you realise that the story isn't going at all where you thought it would be going. So it's not only a great comedy, it's also a great whodunit.