Bottom line is as trials riders the vast majority of the things we ride on don't belong to us. So you're off to a poor start when it comes to dealing with the police.
Don't always assume the police are out to be assholes, it's their job to enforce this. Granted some of them do go out of their way to be dicks, but that's the same with every type of job. Also remember that if a member of public has made a complaint they are duty bound to remove you, they can't turn a blind eye.
Best thing to do is apologise, explain that what you're doing is a sport, tell them you aren't loitering or attempting to cause criminal damage but instead you are training your skills, improving your fitness and trying to further your trials skills.
They will probably still have to make you leave but you'll probably find that in time they might just leave you alone at certain riding spots or not turn up straight away to kick you off.
I've had police here in Norwich kick me off an area a couple times, the last time they kicked me off though they told me they decided to watch me on CCTV for a bit first and were very impressed (which is probably a lie, but it makes you more compliant!).
It's all good in the hood