"No wonder the UK is firmly backing option 1 and persuading all others that it is the only reasonable explanation. There is a lot of political manoeuvering behind the scenes to ensure the "right" outcome."
Rubbish. If there is an airworthiness issue with the airplane, it will take time to sort it out and determine a fix. Unless it is blatant and unmistakable, there would be no immediate airworthiness action taken.
It seems like clues from satellite imagery, and black boxes that a sudden event, consistent with explosion occurred and and enough intelligence clues that a terrorist act may have occurred and it is prudent to take some protective measures in light of the obvious motives against Russia and west European states.
The available clues do not have to be conclusive or even the "only reasonable explanation" to take such steps. They only have to be sufficiently feasible that to ignore the potential for additional loss of life would be reckless and irresponsible.