OK. So we'll go back in history to humour you. Peterloo was in 1819. So that's just shy of 200 years ago. Before meaningful democracy in the UK, as the electorate was massively restricted before the reforms of 1832 (the franchise really started growing after 1867).
Even then, it wasn't a deliberately ordered massacre, so far as we can tell. More the case of not having a civil police force to deal with mass demonstrations, and is probably one of the reasons they created one in the next decade. The death toll was about 15. Note that it was singular, there weren't other massacres, though there was a rather panicked crackdown on dissent. But ten years later the Chartists were allowed to use mass demonstrations and petitions for over a decade - one of the things that led to the 1832 reform act, and that carried on into the 1840s - when the movement petered out.
So even 200 years ago, when the UK wasn't a meaningul democracy, the state didn't use the level of violence that Iran used on the Green movement - who were protesting the rigging of the 2009 vote. That violence resulted in the death of hundreds, systematic torture (including rape) in certain prisions, and far more mass arrests than the British government felt the need to use in the crackdown after the Napolenonic War ended.
So, as I said above. There's no equivalence. Not only with the reactions of our governments now, but even from 200 years ago.