1. Killing members of a group - this can't be a definition of genocide. If it were, then all skirmishes in human history were acts of genocide, lol.
The British Empire did some terrible things - it got involved in slavery on an industrial scale, it took over other peoples' land, it killed natives, etc.
But genocide?
I seriously doubt it. The British Empire, like other empires, wanted to subdue other peoples, take their land, have access to their resources, and trade. It didn't want to wipe them out.
And that is what genocide is, wiping out, or attempting to wipe out, a group of people.