Actually that's pretty much all the officer could do as you can't actually get arrested for civil trespass, the trespasser does have to leave if asked by the landowner, the landowner may use reasonable force to remove them if they resist and they may sue the trespasser if damage to property occurs. In most trespassing cases the police won't even attend unless they believe another crime is likely to be committed. There is a list of places that would be deemed as criminal trespass though (The queen's house, MOD sites, that kinda thing).