as an American, the mere thought of Her Majesty, attacking someone with her purse, sent me into peals of laughter. I don't know the legalities of your propsed situation, but I can make a guess as to the result...
her "victim" would become an instant celebrity, worldwide, make millions in appearance fees, and book deals, and quite generally live a nice life afterwards.
As to the Queen herself, I would imagine her entourage would have her spirited off in no time, and be trying to convince the victim it was in his best interest to keep quiet about it, if it were possible to do so.
Even so, even as an American, I can't imagine anyone trying to bring a lawsuit against her.
Besides the question of possibility, you know her legal representatives would be the best in Britain, right? You would have to have SOME NERVE to face that! lol