That is, what is the limit? What would be transgressing some certain law, and what would be just sufficiently aggressive to get the point across without them being able to arrest? (Or, if they do, they'll find that they have nothing to charge us with.)
I know this would be a generalization, and by default I'm typically friendly with anyone who appears similarly friendly... but, the legal force the police have is also universal (which usually also makes them feel like they have a legal right to be aggressive), so I might as well adopt a generalized, and not naive approach to these kind of people (just like I try as much as possible to avoid chavs, etc... oh, you want to know what those two groups in society have in common, generally? A certain kind of attitude, of course. There may be some intelligent chavs, just like there may be some truly friendly police (and in fact I did meet at least one)... but they do typically fit a stereotype).
And FYI, I want to be aggressive within reason... as e.g. I got stopped (and superficially searched) a month ago by some police, and it was apparently only because it was in the middle of the night and I had a bicycle (they said it's because some bikes have been stolen recently... oh, the caustic irony... when one of my bikes was stolen some years ago they did f**k all), and now I feel afraid to go out in the middle of the night due to that (and I'm afraid to go out in the day due to the outrageous amounts of noise generally present, so I feel trapped). What irks me is that they tried to appear friendly by also asking me what I'm studying at college etc. - this doesn't, though, outweigh the negativity of the fearfulness I now feel. So, if I'm ever stopped again, would it be just fine to say "it's none of your business" if they ask about anything other than names etc?
Just as the limit is to shout at people's faces (and, AFAIK, saying "f**k off" is perfectly legal), what's the absolute limit with the police? Surely I'm not legally obliged to be friendly - as, generally, whoever sees the application of physical force to be a necessity is the last person to deserve friendliness. I understand the police are supposed to keep other groups of people who are similarly in love with physical force at bay, but so far, in my life at least, they happened to ignore me, at best, *every* single time I needed help (although admittedly they weren't typical 'burglar scenarios' - still, completely ignore me when my bicycle is stolen, and three years later accuse me of having stolen one for no good reason?!)