conspiracy of stupidity is more likely
edit:
that actually sounds like the name of a band.
But so many different, and largely unrelated, sources of stupidity are involved though. That's what gets me, it required all of them to come together at once.
There's the stupidity of the invasion of Iraq (which partly precipitated the refugee issue).
The global banking crisis (that removed the healing balm of money, that all-purpose analgesic that soothes all distress, from the system).
The Euro crisis (that made the EU itself look far less appealing and succesful and also drove another unprecedented wave of migration to the unregulated Victorian workhouse that is the UK).
Austerity.
The unnecessarily decision by New Labour to open the doors to the A8 many years before the rest of the EU did (which itself stemmed from the deeply-rooted short-termism of that government, the same thing that meant it didn't do anything to regulate the banking sector - they just did anything that seemed to keep the show on the road a bit longer, with no thought for the longer-term consequences - see also PFI, though that one isn't particularly a causal factor in Brexit).
The stupidity of Theresa May in holding an entirely unnecessary election when she did, and then cocking it up to _just_ the right degree to create paralysis and hand huge influence to the DUP (if she'd cocked it up more comprehensively a majority Labour government might not have gone the same route, as with a decent majority, paradoxically, Corbyn would have had a lot of difficulty keeping his MPs in line).
The idiotic careerism of Clegg and the Orange-booker Lib Dems, who effectively obliterated the one pro-EU party, just in time for this crisis to break.
And the deep structural stupidity in British culture that means we'll tug our forelocks and doff our caps for any group of abject incompetents as long as they went to the "right" schools. Leading to the clown crew of Cameron and his pals holding a referendum with no thought of where it might lead.