Ofcom regulates what is shown on TV and video on demand in the UK. They set watershed times after which more adult content can air. Ofcom's codes of practice are most relevant for determining when a film trailer can air based on its rating. The Advertising Standards Authority regulates advertisements on TV and in sponsored shows, but would likely not be relevant unless the film uses product placement, though that is allowed. Paying attention to advertising rules could allow a trailer for a 15 rated film to air before watershed if it is edited to a 12 rating.