Now that Theresa May has said that it is "highly likely" that the Russians used a nerve agent fairly recklessly as they sought to kill a double agent, lots of questions arise. One could easily code this as an attack, not the first one since other Russians have been killed in the UK, but a more severe one given the number of bystanders and first responders who may have been exposed. And once you use the word "attack," one of the first things that comes to mind is: an attack upon one equals an attack upon all--NATO's Article V. It has only been invoked once, after 9/11, despite other attacks against NATO allies--Russian cyber attack against Estonia, Syrian shells and bombs hitting Turkey, etc.
Consulting NATO via Article IV is a natural way to proceed. It would allow the UK to establish the seriousness of what has happened, that this is a very warlike thing to have happened. However, bringing in NATO involves larger risks. Specifically, the US, which has not acknowledged that Russia is "highly likely" to be guilty here, and with Trump never saying an unkind word about Putin, the danger is that the UK tries to get NATO to take a stand and fails. That would cast doubt about the alliance itself. It is like the US non-response to North Korean missile tests: sure, you can try to shoot them down, but, if you fail, you might reveal that the system does not work so well. Well, with Trump being so hostile to NATO and pretty unfriendly to May, one can easily imagine the US blocking consensus at NATO probably with help from Hungary and Greece. And then what?
With the economic fiasco that is Brexit going on, the UK has not many choices, especially with an unreliable US (today's alliance dilemma horn is abandonment), May is stuck. What can she do? I will agree with those who suggest new economic sanctions, the end of Russian oligarchs moving to London, and kissing RT goodbye. Oh, and, this might sound awful, but the old spy game had some rules, and if one breaks the rules, one should pay. So, yeah, the British should find the most guilty Russian spy, and remove him from the game. Teach the Russians to be more careful.
So far, the Russians have not paid much of a price for their reckless moves across the globe--meddling in elections, using poisons in ways that harm bystanders, and on and on. The US under Trump is not going to act as strongly as it should--the allies will need to do so.
Oh, and when folks say that Trump hasn't made that much of a difference in foreign policy yet, the refusal to respond to the election mess and now this are big pieces of evidence that, yes, Trump is making a difference and not in a good way.