UK thugs use both the carrot (money) and the stick (threats and intimidation) to get protesters to inform on their movements.

We also know that once the thugs have informers in political groups, they don't limit themselves to using the information to stop the occasional real criminal that might show up. Thugs work in cahoots with companies against protesters that criticize the companies' abusive or dangerous practices and even help companies blacklist people that might be "inconvenient".