The poisoning of a Russian double agent in a British cathedral city makes news, but many western politicians will be guided by bigger strategic choices including relations over Syria, Iran, Ukraine and commerce.
Uk has not big trade volume and with EU, UK has already done very much on this side. However UK can optimize the travel ban against the diplomats. Moreover, they can also freeze or create some obtacles against to Russian elites and their firms such as Gazprom and Evraz (russian metal company headquarter in London).
some billions each year go through the British banking system for business purposes. If UK wishes, they may also create some obstacles onthis way affecting Russian economy which is already disrupted due to EU and US sanctions.
It is almost granted that US will ally with UK but it would be hard for EU to stand along UK especially following Brexit. Moreover countries like Germany, Italy and France have economic ties with Russia.
Russia, at least for now, will not do anything else. Basically, Russia will sit and wait. They know something wil happen but they do not what will be. The scenerios are given above. Whatever it would be, UK and US would also harden its stance in Ukraine and put some effort in Blacksea in order to put some pressure against Russia.
