So the Litmitov (sp?) thing was a dry run? I don't remember any type of reaction like this to that incident.
Not at all. It too had the effect of maintaining Russia as a direct threat to national security in the eyes of the British public.

Also, apart from the seriously ill cop who was directly exposed, is there any evidence any of those 500 people are getting sick?
No, it's all theatre.

Maybe the Ukraine or another one of Russia's neighbors did it so we'd be more likely to support them against Russia.
The UK isn't seriously considering this as an option, because if they were, then they would refer the suspect nerve agent to Russian experts to prove it's their agent.

Why would Russia need to be involved in the investigation if it was a third party?
To demonstrate to the Russians that we take the idea of third party involvement seriously. They will want to know it's their agent so they can investigate themselves to see if the Ukranians, for example, could have done this.