I've spoken to a few, made my point and had to call the EA a couple of times when I got rude replies or threatened.
I know in Ipswich the police ejected 14 in one day off the River Gipping.
As for prebaiting on rivers, I don't think it is as essential as on lakes as carp do have to eat more than their lake counterparts, so almost everything is tried.
However, even on rivers baits can blow over time. I have watched River carp ignore sweetcorn for a few days.
Prebait features, overhanging trees, bends, weir pools etc.
Be aware that when you arrive carp may not be in the area, they may patrol up and down a stretch, covering a few miles in a day.
You may find bait droppers handy to get your bait down to the river bed. No point on putting your hook where you dropped bait in if it has been washed down 20metres.