As the resident Brit—and being half Scottish—I should probably take a first crack at that.
Obviously, yes, Britain doesn't have quite the complex federal system that Canada has, but it's not exactly simple, constitutionally, as you might have gathered from watching the news in recent years. Environmental issues are a devolved issue. The U.K. government has to negotiate and co-operate with the devolved administrations of Scotland, Wales and Northern Ireland.
There are other examples of more complex constitutional arrangements that have effective climate legislation. Germany is another example of a federal system—