Brits disliking success starts in school. Think about how we treat the acedemically successful here, compared to many other countries which celebrate success. Kids who are doing exceptionally well in school are often bullied. Think about the way nerds are portrayed in popular culture - often ugly, bespectacled (ideally with braces), socially bumbling idiots.
This mindset then carries on through life, in part fuelled by the very British culture of "understatement", downplaying success and modesty being seen as a virtue, especially in women. Entire chapters in books deal with this mindset, for example Kate Fox's Watching the English (and the mindset does extend to Wales and at least parts of Scotland, too).
Then we have the whole dislike of showing success outwardly. We constantly have to be careful not to boast, which seems to include simply talking about monetary or job-related successes or using social media to "shove our lifestyle in their faces". The whole stereotypical concept of keeping up with the Joneses is built around a desire never to be outdone by those around us.
And all that invites bitchiness when someone does show success. Take MN. There is a thread on here about someone feeling annoyed or even threatened by someone else being obviously keen at work. Another about women's jealousy of each other (which many seem to deny, but I've seen it happen so many times). Another in which someone has lost weight and finds that friends have become colder towards her. My own, which was hijacked by someone with an obvious chip on their shoulder. All in the last day and a bit, and I'm sure there are many more.
I'm not from here, originally, and, having grown up in a different culture, I find it hard to understand why successes are not celebrated. References to luck, outside help etc. being the main drivers behind success - well, that is blantantly misleading. You can have all the help in the world shoved at you, but if you don't take it up and do something productive with it, you will not succeed. Even the element of luck itself, which is merely the ability to look for and take up opportunities as they come. And while the extent of those may differ, we all have those opportunities at some point in our lives.