An attack on the Queen would be seen as an attack on our country.
I think public mourning would be on the same scale as was for Diana, because although the Queen is regarded in a completely different way to Diana, it would be an undeniably deliberate killing, and that's what would hit people the most.
Likewise, although 9/11 and a hypothetical assassination of the Queen are incomparable (many deaths instead of just one), I think national and international shock and outrage would be of a similar kind.
If a recognisable group, whether religious, national or otherwise, was responsible (or even suspected as being responsible), I think there would be reprisals (especially if members of that group were in the UK).
Anyway, what on earth made you think of asking this question?