Well... there are reasons for that. The American population is much higher than that of the United Kingdom, and the crime rate is also higher there. The United Kingdom does not have high-crime inner city areas (although there are indicaitons that ethnic gangs are about to create them in places such as London, Bradford and Oldham), and does not have to deal with porous borders.
even allowing for population size and ethnics there's too wide a discrepancy, there's a big cultural difference, many american police expect to be treated like gods and all they need to say is "he wasn't complying with my orders" to get away with killing someone
