It’s not about numbers, it’s about people. You are correct, booting the royals out of their palaces and turning it into housing would not solve the entirety of the UK’s homeless problem, and while I’m guess you made up the percentage, housing for 6950 people currently unhoused or insecure living conditions is a better use of the buildings, which already exist. The royals also have multigenerational wealth that isn’t tied to stolen items you could transfer to agencies that serve the unhoused, build new developments. Even if didn’t use this moment to overthrow the entire capitalist system (which is unlikely) that wealth could be used to help the impoverished take steps within that system to get affordable housing.
And yes, wether symbolic monarchists holding on to the spoils of the previous centuries or the post-industrial/digital revolution corporatists that have risen in the last couple, the wealthy should be afraid that at some point the bulk of society is going to come kicking down the doors of their palaces, mansions, penthouses, and maybe even engage in a bit of piracy when they flee to their super yachts to reappropriate their estates for distribution to the masses. The world doesn’t need kings and billionaires.