In 2012, Mr Assange was arrested and bailed. He broke the conditions of his bail when he entered the Ecuadorian Embassy, and following this the Westminster Magistrate issued a warrant of arrest against him. Thus the police are obliged to arrest him if he leaves the embassy. The offence of breaching bail on conviction normally leads to a prison sentence, of up to one year.
If he was arrested, the USA would almost certainly immediately issue an extradition request, and given his previous actions, Mr Assange would not be bailed again.
And hence the standoff continues, but as the charge is no longer one of rape, but of breach of bail, there are far fewer resources being put in his apprehension.
This is analysed by the Guardian in May.