It's perfectly legal to walk into someone's house (if they've left the door unlocked) as long as there's no intent to steal, to cause damage or to harm someone. Trespass is normally only a civil matter, not a criminal one. (There are certain exceptions, relating to such places as railways or military establishments, but they don't relate to private houses).
Similarly, there's nothing to stop you getting into an unlocked car. (There is a law about 'interfering' with a motor vehicle, but that comes under the Criminal Attempts Act 1981. If there's no attempt to commit a crime, there's no offence committed).
Chris