I believe it's worse because firstly, a lot more people died, and secondly the Brighton bomb was an attack against a government rather than an attack against ordinary civilians. I'm not saying the Brighton bomb was right, it was deplorable.
Similaly with 9/11, most people think of the twin towers. Lots less think of the attack on the Pentagon because that could be constituted as a legitimate military/government target, even if it was a plane full of civilians which was used for the attack.
Ultimately, I'm with LP on this one. Does America's foregin policy mean that we should remember or grieve for those civilians who died and their families any less? How would you feel if a terrorist had killed someone you knew? Would it be any more acceptable just because our government's foreign policy is bullshit?
Had they lived, many of the victims of 9/11 probably would have been against America's role in Iraq. America is asking for it a lot of the time with its foreign policy, as is Britain to a degree these days. But targetted attacks designed to murder as many ordinary civilians as possible? Regardless of whether it's Al Quaeda, America, Israel, Iraqi insurgents etc, it's never even remotely condonable.