Spy poisoning: How could the UK retaliate against Russia?

Spy poisoning: How could the UK retaliate against Russia?  BBC News UK's May says 'highly likely' Russia behind nerve attack on spy  Reuters The UK prime minister says Russia tried to assassinate an ex-spy on UK soil  Vox 'Highly likely' Russia poisoned former spy, UK's Prime Minister says  CNN Newspaper headlines: May's Russia ultimatum and 'eggstortion'  BBC NewsFull coverage