[dragonwriter]

There's no violation; the “privileges and immunities” referenced are those of accredited diplomats to the receiving state, which you only get to be by consent of the receiving state. Assange was never an accredited diplomat to the UK (there was an abandoned effort to get him appointed as a diplomat to Russia, but that provides no mandatory protection under the convention with regard to the UK, and the UK declined to extent diplomatic status as an non-mandated courtesy based on that appointment, so it was abandoned.)

> However, if the UK had the legal right to refuse passage for non-embassy personnel then surely they would've done so and captured all non-embassy personnel for questioning.

Every person exiting the embassy was stopped, frisked, questioned, and photographed by the police; none were held further because they were all diplomatic personnel. (This is not addressing the people who exited before the police through a complete cordon around the embassy, or the material—including certainly he weapons and other evidence—sent out in sealed diplomatic bags thereafter and before the final evacuation of the embassy.