What if you are wrong in your assumption that Iran will not export its violence and its nuclear demons? The Islamic Republic is already the primary supporter of Hezbollah and Hamas, who have demonstrated repeatedly their willingness to slaughter Palestinians to feed their hatred of Israel. The Islamic Republic is the nation that denies the Holocaust, and that would that Israel cease to exist entirely.
Regardless of what sins the US and the UK have committed in the past, how long until standing idly by becomes a more grievous sin?
Acting now may be premature.....but what consequence if acting later is acting too late?