In terms of GDP, the United Kingdom is still the 6th or 7th richest in the world. That's still pretty rich.
There are also a lot of wealthy European countries that had a minimal presence abroad in the 19th century. There doesn't appear to any correlation between the modern wealth of western countries and their past colonialist ambitions.
Agreed, but the original poster didn't mention standard of living, only how "rich" the country was. That sounded more like overall GDP than GDP per capita to me (saying "Luxembourg is richer than the USA" sounds a little wrong, right?), though it admittedly it was just a guess as to what he or she meant.
It seems to me the GP didn't talk about the relative wealth of e.g. Britain, he was expressing a conspiracy theory that everything Britain had was stolen?
It took me a bit to realise that you did answer the GP's "point".
Several former British colonies are now wealthy, first-world nations e.g. Ireland, Australia and America. Being an empire at one period in history would explain why a country was much wealthier than others, but not anymore - now it's trade. Just look at the rise of China.
(Some might argue that America is a de facto empire today, but even then most of its wealth is due to trade).
And they aren't a very rich country now, after they lost all their colonies.
I don't wish to be harsh here, but how much of west's wealth is their own hard earned through their sweat is a question to ask.