I absolutely have acknowledged that even highly trained horses can spook. Read back again and you'll see it.
You're the one who has failed to acknowledge 1) that these horses are very highly trained and that their training includes shouting, screaming, having things thrown at them and having things waved at them, and 2) that these horses are just as likely to spook at things like a large union flag waved by an enthusiastic Royalist, therefore the risk is there no matter what.
You are also the one who, by your own logic, thinks that protests shouldn't be allowed because of things that might happen.