First of all, there is hardly any evidence to prove that it was Russia which was behind the poisoning of Skripal and his daughter in London. And secondly, this is something that the western agencies such as the MI6, CIA and the Mossad has done in the past.
I would even add, if I may, that there is hardly evidence that any real poisoning has happened at all. What do we know in fact? Just what we have been told by very few sources. And for sure, if the poison would have been the one which has been mentioned, nobody could have survived. But they survived.
That's an important point to be noted. According to the latest reports, both Sergei and Yulia are conscious and reacting well to the medicines. If the FSB was behind this poisoning, then these people would have been dead in a few days.