I doubt anyone knows for sure, including Iran. Iran has a lot of decentralized proxies over which it doesn't have full and direct control. The official Iranian government would probably try to strike some symbolic blow, perhaps by hitting Jerusalem or Tel Aviv with a missile, or shooting down a few planes or ships. It could certainly shoot enough missiles at Israel to kill a few hundred Israelis. Hezbollah would probably shoot a few hundreds rockets at Israel. Other local groups might attempt minor measures against Israel. Hezbollah-affiliated individuals abroad might attempt to assassinate Israelis/Jews abroad, perhaps in the US or Britain. I don't think Iran is so stupid as to directly strike the US or Britain though, as that would lead to the kind of escalation that might lead to the overthrow of the Iranian regime.