What an amusing story. Are you sure that the police were not provoked into action? Are you sure that you hadn't drunk enough of the bottles contents to not really be aware what you were doing? Did you infact throw said bottle at the police? ay? ay?
Seriously though, it's illegal to have open alcohol in public there? Wow, land of the free, huh?
Oh yeah, about the unmarked cars as well, anyone in England who's ever driven on a motorway for more than an hour will have seen an unmarked police car at the side of the road having pulled someone over. Not just Focus' though, although the Focus ST unmarked police cars look AWESOME, there are many many different ones they use here, including old bangers, I've seen someone pulled over by an old Ford Escort RS Turbo, so be very aware if you come to England