The view of the Supreme Court of the United Kingdom was that the "interview" was, in practical terms, equivalent to arrest under English and Welsh law. From memory (though this was years ago!) a formal charge must be occur at the end of an interview. There's no distinction, in the view of the Supreme Court, between the interview and the arrest.

The argument that he's only wanted for interview was a large part of his defence prior to entering the embassy, and was entirely shot down by the courts.