Im afraid to say that I know of people who haven't been able to claim on insurance for accidents involving their horse's when they haven't been wearing the correct gadgets.. namely bridles.. they've taken their horse for a walk in a headcollar and lead rope, the horse has bolted for some reason or another, and ran between cars and through gardens.. they had to pay out for the damage, as the insurance refused to pay out..
As for correct clothing, I heard a about a year ago, that insurance companies will not pay out for accidents if the horse or rider are not wearing at least 3 illuminated items between them (bib, wrap over for horse, or boots etc)..