Re: How would they have hurt anyone in the UK ?
Strikes me that both Libya and Iran financed and supported the IRA. But nobody I know blames them for the Troubles.
If ISIL were not an expansionary movement desiring world domination and/or bringing about an apocalypse -- if ISIL were "merely" a genocidal horror like the Khmer Rouge in Cambodia - then ignoring them as "somebody else's problem" might make sense. Sometimes the only choices are "terrible" and "even worse".
But ISIL is an expansionary movement, so if it's not dealt with now, it will have to be dealt with later, and the cost will be far greater. Neither is any compromise possible with a movement that glorifies rape, slavery, torture and genocide.
As for the IRA: It was possible to negotiate (some would say compromise) with people that shared its aims but not its methods. Irish Republicanism is not morally repugnant if pursued via civilised political debate. Also it was the USA that was the worst culprit when it came to funding the IRA. (People living in the USA, not the USA government, except to the extent that it was at that time unwilling to interfere with their ability and rights to legally send money to the IRA. 9/11 changed that in an instant. One of history's ironies: it was Al Quaeda that broke the IRA )