there are a number of plausible reasons.
1) they don't actually know how or where the nerve agent was administered, so they are proceeding with an abundance of caution (i.e. better safe than sorry)
2) trace elements were detected in public places (brought by the Skripals, allegedly), which also links neatly back to point 1
3) taking a page out of the Putin playbook, May is trying to drum up nationalistic fervour to draw support for hitting back against Russia. the greater the public outrage the more support she will have for extra sanctions, Magnitsky style law, etc. plus, trying to stir up a nationalistic frenzy is one of the better known tools in the political playbook for rallying support.
I've answered your question. now your turn.