Quick synopsis without giving the game away:
Basically we follow a bunch of chavs (bad-bois, layabouts, whatever you call them...) as they proceed to mug a woman and then get attacked by aliens. The police are stumped.
That pretty much is the jist of the film.

I got this through LoveFilm and absolutely loved it. Granted the storyline is somewhat non-existent and it was kind of annoying that they put the characters in such a situation but the characters are what make the film great.
I watched it with a mate (he live in London by the way) and both of us were cracking up.

The fact the film was made to be a comedy with some great 'tongue in cheek' action made for a very good film and both of us came away saying this was the best film we'd seen in a long time.
If it didn't have the likes of Nick Frost in and two 5 year olds trying to be 'bad-bois' I don't think it would have been good at all. It reminds me a lot of Kidulthood to a certain extent but without the drama and complex storyline.
Oddly enough the film does have a bit of a parable embedded into it which was a bit odd (basically don't f**k with aliens)

Is it worth watching? Yes. It is worth buying? Absolutely. I'd suggest its best watched with friends though, I think you'll get a better comedic effect with added banter.