This will be, beyond any argument, a blatant breach of the Vienna Convention of 1961, to which the UK is one of the original parties and which encodes the centuries - arguably millennia - of practice which have enabled diplomatic relations to function. The Vienna Convention is the most subscribed single international treaty in the world.
The provisions of the Vienna Convention ( http://untreaty.un.org/ilc/texts/instruments/english/conventions/9_1_1961.pdf) on the status of diplomatic premises are expressed in deliberately absolute terms. There is no modification or qualification elsewhere in the treaty.
1.The premises of the mission shall be inviolable. The agents of the receiving State may not enter
them, except with the consent of the head of the mission.
2.The receiving State is under a special duty to take all appropriate steps to protect the premises
of the mission against any intrusion or damage and to prevent any disturbance of the peace of the
mission or impairment of its dignity.
3.The premises of the mission, their furnishings and other property thereon and the means of
transport of the mission shall be immune from search, requisition, attachment or execution.
Not even the Chinese government tried to enter the US Embassy to arrest the Chinese dissident Chen Guangchen. Even during the decades of the Cold War, defectors or dissidents were never seized from each other's embassies. Murder in Samarkand relates in detail my attempts in the British Embassy to help Uzbek dissidents. This terrible breach of international law will result in British Embassies being subject to raids and harassment worldwide.
The government's calculation is that, unlike Ecuador, Britain is a strong enough power to deter such intrusions. This is yet another symptom of the "might is right" principle in international relations, in the era of the neo-conservative abandonment of the idea of the rule of international law.
The British Government bases its argument on domestic British legislation. But the domestic legislation of a country cannot counter its obligations in international law, unless it chooses to withdraw from them. If the government does not wish to follow the obligations imposed on it by the Vienna Convention, it has the right to resile from it - which would leave British diplomats with no protection worldwide.
I hope to have more information soon on the threats used by the US administration. William Hague had been supporting the move against the concerted advice of his own officials; Ken Clarke has been opposing the move against the advice of his. I gather the decision to act has been taken in Number 10.
There appears to have been no input of any kind from the Liberal Democrats. That opens a wider question - there appears to be no "liberal" impact now in any question of coalition policy. It is amazing how government salaries and privileges and ministerial limousines are worth far more than any belief to these people. I cannot now conceive how I was a member of that party for over thirty years, deluded into a genuine belief that they had principles.
[Side note: Craig's site has been overwhelmed by traffic, so reposting here - repost and tweet at will]