You may call it self-harm, but I think we need to consider Britain's relatively pragmatic / less than enthusiastic membership of the EU. For a start, Britain has a completely different legal system to the rest of Europe (common law vs. civil law). That was bound to create tensions.
I don't see how you can make such a binary claim - UK vs "the rest of Europe". Your map shows 6 broad categories of legal systems in use across Europe. Even if you put the 5 non-common law systems into one bucket, it still wouldn't make the UK unique as Ireland operates under common law.
> Would the former British colonies be doing better or worse had they never been colonised?
It's not necessarily correct to assume that they failed because they were previously colonised by the British. Should we have instead allowed other powers to colonise them? That is the only realisitic counterfactual here. Can we truly say the world would have been better off if the British had not colonised various countries, for example when it came to waging the Second World War?
https://commons.wikimedia.org/wiki/File:Legal_systems_in_Eur...