Battle Royal you say?
Well, I'd probably have to tackle the older Royals first, Prince Philip etc, but avoid the Queen herself, she looks very feisty and her bodyguards would probably be protecting her anyway. After the older ones are down, I'd move onto prince Harry/William, make sure they can't retaliate when I try to scare off their missus (well, just Kate really).
Finally, after I've knocked out Camilla, the Queen and I would Hadouken each other until someone goes down. Probably. I dunno, she seems like a Hadouken spammer to me.
I mean, do you see it going any differently? I don't really see the point of this thread, everyone know the Royal Family are the Royal equivalent of Dragon Ball Z, they would straight up kick anyone's ass. Especially if they're foreign-looking.