The UK population was so willing for at least 30 years following WWII, to a large extent thanks to the experience of necessary cooperation among the generation who fought that war. But over the *last* 30+ years that willingness has been steadily eroded by many factors, including (but by no means confined to): greater individualism stemming from precisely the relative affluence and economic freedom that post-war social democracy conferred; successive economic crises (some related to oil, some to financial recklessness); industrial decline, outsourcing and austerity imposed by politicians in thrall to neoliberal economics; free market propaganda promulgated by politicians in thrall to neoliberal economists; mass migrations; international terrorism.
The UK Brexit referendum, US election of Donald Trump, and developments within many EU countries suggest that this willingness has now been lost by somewhere around a crucial 50% of my own "baby boomer" generation, and there's evidence of loss too among younger generations whose expectations have been drastically curtailed. But despite the nationalist rhetoric of "taking back control" from the Brexiteers, it seems more likely that what's actually happening is a withdrawal of people's engagement from the nation-state altogether, back to the individual family as unit of survival.
Perhaps the only way the willingness required for social democracy could ever be restored is in the event of some major catastrophe, on the order of magnitude of a world war, great depression or an abrupt climate deterioration, that forces people to relearn cooperation in order to survive. Recent governments in both Europe and USA have been just barely prudent and competent enough (tempering their neoliberal policies with judiciously-applied shots of Keynes during the emergencies) to avoid such a catastrophe. Such a catastrophe feels quite a lot closer following the inauguration of the impulsive President Trump, but a catastrophe it would remain - and to imagine otherwise would be grotesque.