I'm an American. I enjoyed watching Criminal Justice but I was really surprised by a few things...
Do police and corrections officers actually treat criminals with kindness and respect? For example do they actually say please and thank you? That would never happen in the USA. Cops would rather beat your head in than show an ounce of decency to a criminal.
Also are british people really that nice in general? Even in big cities?
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Your film gods: Lee Van Cleef and Laura Gemser
http://tinyurl.com/pa4ud44
My house was "raided" a few years back in a drugs thing and the police were very courteous. They already had a warrant so they could've broken the damn door down if they wanted but they knocked and asked politely to come in. Far from trashing the place, they carefully looked through the house. They were friendly when conducting the search and taking statements. Really it was hardly like they were looking for criminal activity at all. I can also attest, albeit secondhand, that their treatment of people in custody is generally good as well. Nobody that I know has experienced mistreatment (even one of my friends who was falsely accused of rape).
I can't speak for everyone but it certainly seems that, on average, British police are far more polite and respectful than their American counterparts. They don't stay that way if you get nasty with them though. In my experience, British police treat you the way you treat them.
As for the general public, it largely depends on where you go. I've lived in London and it's got that big city attitude of everyone keeping to themselves. It's not friendly but it's friendlier than a lot of big cities around the world. Londoners, for the most part, are polite enough; they just never talk to each other. Unless you're in the very worst parts of London, you won't find trouble unless you start it. I've lived in and spent a lot of time outside at night in some traditionally "rough" parts of the city and I've never even been mugged.
Compared to America (where I've visited multiple states), I tend to find that there's a lot less false or superficial friendliness in Britain. I hope you don't think this is rude but I feel like the US has something of a culture of insincerity in social interactions. Some of the people I spoke to in America gave off the vibe that they felt obligated to be nice and friendly. The result is that it's harder to read people's true feelings and fully connect with them. I imagine that, living there, the phenomenon would make it harder to have real, tight friends-for-life relationships with people. In Britain, people are prone to saying what they mean and meaning what they say. You don't make as many friends but the friends you do make are the kind you trust implicitly. Some Americans who came over here tell me that they initially interpreted us as cold but eventually realised that our social interactions are just more organic.
Polite? Very often. Nice? Mostly. Friendly? Sometimes. Sincere? Definitely.
I hope that was somewhat enlightening. It's a good question because you should never base your opinion of a place on what you see on TV. :)