Unfortunately, I only passed the Briton’s Protection on my last day in Manchester, on the way to the train station, with no time for a pint. But I was interested by the sign, showing soldiers on horseback chasing down people carrying ‘REFORM’ banners. I took a picture and looked up the story behind it when I got home.
It refers to what happened at nearby St. Peter’s Field on a summer afternoon in 1819, when around 80,000 people gathered to peacefully demonstrate for voting reform. The British Library site has a good summary of the context and what happened that day.