The point is that if someone is going to be in the wrong place at the wrong time, they are no more likely to end up near a VIP than anybody else, and the tiny number of royals flying around makes a mid-air with one of them even more exceedingly unlikely than is already the case (very exceeding unlikely) anyway.
All this does is that it wastes ATC resources managing this stupid Class A, for the benefit of those few who actually go to ais.org.uk and do a Narrow Route Briefing, while conveniently announcing the very spot to anyone who might want to get close deliberately.
IMHO it is a very good job the ais.org.uk website didn't come on line while the IRA were still properly in business.
I am not anti-royal; I just think the temp airspace is a complete waste of time. They'd be far better off doing an unannounced flight, in or out of CAS.