>By many measures the Roman and British empires were 'more successful'.

With the Roman Empire you are overlooking their slavery, genocide, etc, most of your critique applies. Britain at least outlawed slavery at home, but not in territories abroad, hence the slavery in the Americas and elsewhere

> Britain at least outlawed slavery at home, but not in territories abroad

But...it didn't. I mean, not if "at home" means "throughout Britain" rather than "only in England and Wales".