If they're just silently holding a banner or turning their backs or something then fine.
But if they're disturbing the solemnity of the occasion for other people that's just twatish and I have precious little sympathy if the letter of the law is stretched. Whether you agree with the monarchy or not is irrelevant, people have come out to pay their respects to a deceased person and there's a family who are having to do their grieving in public. Not the time to be abusive or seek publicity, peacefully or otherwise.
The irony is that the kind of people who would spoil this for others would be the first to kick off if you dared disrespect their family.