An angle that seems to be forgotten here is the political bind the UK government finds itself in. On the one hand, they cannot afford to violate territoriality of the Ecuadoran embassy (legally or not) - on the other, they also cannot afford to break the terms of their extradition treaty with Sweden.
Either outcome will see the UK have further problems on the world stage, and neither is something they can easily negotiate their way out of. In such a situation, it might be imagined that they would rather risk their reputation with the rest of the world than alienate their allies.
It's not really a bind. They could just as easily say "Sorry, Sweden, he made it to neutral ground" as "we're going to violate the Vienna Convention". At this stage it's all politics; if the UK is threatening to do this, then it's getting something out of the deal.
You may be right, but I would be a little concerned about what form those "further problems on the world stage2 might take if I was a UK diplomat, or, worse, a local civilian employee, in a UK embassy somewhere interesting.
Either outcome will see the UK have further problems on the world stage, and neither is something they can easily negotiate their way out of. In such a situation, it might be imagined that they would rather risk their reputation with the rest of the world than alienate their allies.