Ich zitier mal aus der Wiki:
Watershed is a term used to describe a time in television schedules during which it is permissible to show television programmes which have 'adult content'. Adult content can be generally defined as having nudity, explicit sexual intercourse, graphic violence, strong language, or drug references or use.

Bezogen auf UK:
According to Ofcom, the watershed on standard television in the UK starts at 9:00pm, and finishes at 5:30am the next morning. On premium film or pay-per-view services requiring a subscription, the watershed starts at 8:00pm. However, 12, 15 and 18 rated films can be shown on pin protected channels (such as Sky Movies) at any time of the day. Viewers are required to enter their PIN to view. There should be a gentle transition to adult material, and 18-rated material is not allowed to be shown before 10:00pm