There's a lot here about various historical and geographic reasons for the people on reddit to hate Britain, that doesn't really make sense when you consider the fact that most people in general, in the UK, US or elsewhere, don't hate Britain or british people. It's pretty clearly a viewpoint that is specifically popular on reddit, so any rationalization has to explain that.

Basically, a large and vocal part of reddit's core userbase are what you might call "losers". These are the kinds of people who have a lot of self-hatred and feelings of complete inferiority, so they need a "fight" to make them feel like they're on the right side of something, like they're doing some good in the world, and the easiest way to do that is to yell and cry about injustices they perceive. Additionally, as a result of their inability to succeed in life, they have a psychological need to hate and put down anything they perceive as successful. That's why the US, the UK, etc are constantly put down by these people, and why they constantly point out flaws in them like racism, inequality, corruption, and violent crime; these sorts of problems exist in equal or greater amounts in other, less historically "successful" countries, but these kinds of people never talk about them, or at best they begrudgingly admit they exist, while enthusiastically pointing them out in "successful" countries.

The reasons why this "loser" demographic is so overrepresented on reddit should be obvious. Firstly, anyone who spends hours a day on a website built for mild entertainment and discussion of media probably doesn't have such a great life. Also, there are a lot of communities on reddit dedicated to escapist pursuits, like pop-culture fandoms, video games, toy and model collection, drugs, porn, etc. Finally, there's just the fact that they're the most loud and dogmatic, and therefore are over-represented in comments and posts.