Yes... in many places park gates are locked at dusk - to save the cost of policing them I suppose.. if anyone breaks in and damages something then they get charged with trespass as well... there are very few truly 'public' parks in London, they're owned and maintained by local borough councils and estates so can be quite legally shut at night or an entrance fee charged for events and displays. Only 'Common' land cannot be fenced off and an entry fee charged.

LTS