Recent imperialist ventures - Iraq comes to mind - where the US and its allies overthrew the local dictator - have led to a worse mess.
Let's not forget the long-term effects of British rule in places as far-flung as Ireland, Cyprus and - of course - Iraq and the rest of the Middle East. A policy of divide-and-rule led to pretty awful consequences.
British government during the 18th and 19th centuries was spectacularly corrupt - the rotten boroughs and the Opium Wars being two examples.
Where decentralized British rule "worked" was where the Brits deliberately or accidentally wiped out or effectively displaced the native populations - Australia, Canada and New Zealand.
(Disclaimer - I'm British, and have reasons to be proud of my country, but our Imperial past is not one of them).