In this video I think it's demonstrated incredibly powerfully what importance needs to be placed upon anti-horse tools.
The horses do the bulk of the work in creating space that allows the police line to tighten the noose here. Without them they never would have controlled these crowds with people on multiple sides of them. They could not have maintained control.
There must be simply and effective methods to spook these horses without causing a danger to them. Something that could be deployed that would make the riders tell their superiors "we can't deploy because the horses would be spooked by x".
Anyway. What are other people's thoughts here? Obviously this protest was unprepared for a fight (although several police have been hospitalised). What could small groups of 1-5 have done in the wider engagement to make things go differently?
Anything you do to a police horse is likely to be treated the same as attacking a human cop so don't do anything below.
Under a horse's neck about where it meets the chest is usually where I put my hand when wanting to push a horse backwards while it is facing me.
You can try grabbing the bridal/reigns while you're up close and then you should have more control of where the horse's head is pointing (and where it more likely to want to move) than the rider.
As has been noted in several other comments, they should be "bomb proofed" so things like loud sounds and flashing lights shouldn't spook them. Might need to try something high intensity and directed. Lasers or acoustics, probably not a great look as our out animal rights comrades will point out. Olfactory might be useful. Sometimes blood or rotting meat will upset our horses. That could be an option. Female horse piss while they are ovulating might be enough to agitate a gelded male horse, pheromones and what not.
If you can do something to the surfaces that horses are expected to walk on... gravel, sludge, oil... anything that will cause a horse to balk at the loss of traction. This isn't guaranteed as some horses are so dead broke you could ride one off of a visible cliff and they wouldn't care.
Narrow pathways and object of a similar size to the horse can sometimes be enough to make them shy away from moving in that direction.
If the mounted police are using verbal commands to their mounts, pay attention to what they say, the volume and inflection. The horse will likely try to obey any command that sounds like the command that their rider is making.
Riding crops and lunge whips. Common training tools that the horse is likely familiar with and will respond to even with a rider on it's back. Crops are the short sticks with a flappy bit at the end that can make a satisfying pop when it hits something. Easy to carry, easy to use, hard to hurt yourself or others. Lunge whips are much longer sticks (4~6 feet long) with a 4~6 foot length of rope on the end as the whip. Makes a much louder noise but takes a whole lot of space around you to use without smacking people around you.
If there is prep time, any artists that can paint crazy geometrical/off perspective shapes on the ground to make it look like the flat ground has holes and/or constant changes in elevation could help to confuse a horse as to how confident it can be when taking a step.
Oh, almost forgot. Lead ropes. Thick cotton/sythetic fiber ropes with a large spring clip on the end. If you an get close enough to clip the rope to the ring under the horse's chin or on the right/left side of the jaw you now have a way to pull a horse around and possibly to the ground with a group of people.
I wonder, could you do the crazy geometrical shapes on banners? Like "dazzle" camouflage, or some optical illusions? I'm not sure if being on a soft fabric like a banner would yield the same result.
Or on tarps, to combine with @Awoo 's tarp thoughts below?
Possibly the bomb proofing that horses should have gone through might have given them some resistance to things like fluttering banners.
But anything that would make distances hard to make out for a prey animal that doesn't have forward binocular vision. So if the banner patterns somehow worked with the surrounding landscape to make it look like a wide two lane road was barely one horse width wide might help to confuse the animal about how much space it has to work with.
Second but... any pattern on a banner that could be carried and then deployed on the ground to make a solid roadway look like there is a huge hole in instead might be worth the effort.
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This is some Wile E Cayoty shit
Never been to the UK, but how are cross walks painted on the ground? In the USA they typically are wide horizontal bars, white or yellow. So if the ones in the UK were of a different pattern, I'd think that painting cattle grid/gards (roughly a series of wide rectangle shape) on the ground with might be feasible.
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Wile E Coyote has entered the chat
Simply paint a tunnel in the wall and have the protestors walk through it