Oh my. Well just off the top of my head. The UK has committed widespread systematic war crimes/crimes against humanity for generations in Ireland (+ is responsible for the deaths of the famine), Kenya, India, South Africa and localised war crimes in Iraq, Afghanistan and many Asian countries. Bombing of Dresden also. To name but a few. Then you can probably argue forcing and dumping opium on China is another one.
https://en.m.wikipedia.org/wiki/British_war_crimes
Even today that are recent British war crimes where the perpetrators are alive and yet will never be prosecuted.