Indirectly, pressure from UKIP led to the current Brexit situation—which as gjm points out, has not yet resulted in the UK leaving. However, UKIP’s vote increase didn’t cause Brexit, it simply led to a referendum. But the conservatives could have easily not called the referendum in the end since UKIP’s high vote share did not translate into any seats in government. I think it’s much easier to pin the blame on Cameron’s arrogance and putting party politics ahead of country, than it is on UKIP.
Nonetheless, even if you do not agree with that assessment, Brexit remains one data point. I am not personally aware of any other such events occuring in the likes of Canada or India, or similar examples in the UK.
I know little of the US pork barrel system, so can’t offer comment on that.
The Coalition government of 2010 was the first coalition in almost a century, and indeed you can find only 2 coalition governments in modern British political history. It was not the norm at all, and was heavily influenced by the bizarre regional nature of British politics. Meanwhile the most recent election can be seen as another FPTP aberration, since the most representative government would have been a Lab-Lib-SNP coalition rather than a Con minority. What is more, 2017 saw minor parties almost completely wiped out with 82% of votes and 89% of seats going to the 2 largest parties. The UK is a poor example of third party influence, I would say.