I don't - my father's funeral is literally two weeks away. Some relatives that my mother doesn't get along with at all aren't invited - they've never been particularly close and it'll be healthier for everyone if they don't stir shit in a time of healing.
This isn't a case of protestors being repeatedly ignored - the public discussion about the monarchy is unfolding in most media outlets right now and other forms of protest are still quite available.
Sorry about your father. I don't like using personal examples since emotions can be strong, especially at a time like this. If they were to show up and stand peacefully with a sign, I don't think that should be illegal. But if they did that it would certainly show the rest of the family what kinds of people they are.
I mean - whenever we're talking about funerals it's an extremely personal event for some. Just because you can't see the person - or it's a person of privilege - doesn't mean it doesn't hit just as hard.
Additionally, while I think this is extremely unhealthy mentally, there was a not insignificant proportion of Britain that felt they had a personal relationship with the queen even if they never met her for tea.
Protesting a public funeral procession for a public figure down closed-off public roads makes that more defensible, from my Australian perspective. Otherwise you have a bizarre situation where you can hold a 100-mile-long walking-speed parade for whatever cause you support, provided you can find a suitable dead person and convince the police to block off the roads, and all your opponents have to shut their mouths and put up with it.