Getting the components needed for a nerve toxin isn't easy. In addition to the specific expertise and equipment needed to produce it, you then have to get it into the UK, because there is no way you can import those components without some very heavily scrutinized import licences.
Beyond that, it's a terribly expensive way to kill someone. A guy with a gun, or a knife, is always a sure bet. So why do you use a nerve toxin, or polonium or similar? To make it clear it was you, without saying it was you, and to send a message. That message is you will die horribly and in pain and it will take weeks, and you deserve it for being traitor scum.
As for why this doesn't appear to have killed them, there could be a number of factors. Dispersal may have caused a lower than lethal payload to be administered. The spy and his daughter were found relatively quickly, and nerve agents take time to work (they make it so the body cannot produce important enzymes, leading to the eventual breakdown of the body's ability to maintain itself). Nerve toxins also appear to be hard to use, the Tokyo sarin attacks should have killed hundreds but only managed to kill less than 10 because dispersal lessened the impact of the sarin. It may also be a new compound, which would explain why they are cagey when it comes to naming what it actually is.