The police in the UK can be very different from those you encounter in more 'shoot first ask questions later countries'. Reading the OP again just now it doesn't sound all that unbelievable to be honest, I used to have a friend in the Met and he would often tell us stories about confiscating small personal quantities of drugs and disposing of them to avoid the paperwork and unnecessary disruption to someones life over an 1/8th of hash. I still wouldn't have offered up the gram as claimed by the OP though but if this is true I can understand where the motivation may have come from to be up front with them, UK police can actually be reasonable at times, something I am yet to experience in Australia where they are mostly just a bunch of power trippin' wankers. Still the best advise always will be do not talk to the police!