Assange cannot be arrested in the Ecuadorian embassy without the permission of the Ecuadorian ambassador. The British police can't enter the embassy without permission. The balcony makes no difference.
> That norm is routinely violated in other places
No, it isn't. That would severely damage relationships between the two countries. It's more than a "norm", it's a fundamental principle of diplomacy going back millennia.
It's a fundamental principle of diplomacy going back 55 years, and if you'd like to put money on my inability to cite you an example of a country not respecting the inviolability of a diplomatic mission on their own soil, I invite you to name a number.
At any rate: we agree: it's the Vienna Accords that keep the UK from simply lassoing Assange and hauling him out of the building. It has nothing to do with whether he's on Ecuadorian soil (he is not).
The Vienna Convention is only the current basis for international law. The concept of diplomatic immunity is at least as hold as history itself. Yes it gets violated occasionally, like nearly every law and custom we've ever created. That's generally treated as an act of war though. You're the only one in this thread that said anything about Ecuadorian soil.
> That norm is routinely violated in other places
No, it isn't. That would severely damage relationships between the two countries. It's more than a "norm", it's a fundamental principle of diplomacy going back millennia.