Why didn't they get done for public indecency? (uk law)?

my friend and her boyfriend had drunken sex at 4.00 in the the morning when no one was out.. it was in a park and nobody saw... except a policeman who happened to see something when he was driving pass. He stopped them and told them to cut it out, then got back in his car and drove off. Why didn't they get done for public indecency. I'm in the UK by the way.