It's not easy keeping track of exactly who has promised what to whom in the race for the Democratic nomination. Two of the most prominent candidates, Sen. Bernie Sanders (I–Vt.) and Sen. Elizabeth Warren (D–Mass.), have made "free" college and student debt forgiveness central pillars of their presidential bids.

But the U.K. has moved in the opposite direction, with astonishing results. After nearly 40 years of offering free college, starting in 1998 students in England were asked to pay about $1,200 per year. Since then, tuition has risen to $11,000 per year. Students in England now leave college with around $60,000 of debt on average. A government-backed loan scheme means graduates only start repaying that debt once they are earning more than $30,000. Any debt that remains after 30 years is forgiven altogether. (The Scottish government has devolved power in this area and operates a different system.)

Yet today the number of university students from poor backgrounds is larger than it has ever been. The story of why the British government opted to end a policy similar to those proposed by leading Democrats—and what has happened to higher education in the U.K. since—is instructive for the American debate.

The case for free college is simple enough: higher education should be for the many, not the few. In the U.S., today's eye-watering costs prevent that from being the case. Sanders called it "a national disgrace" that many Americans don't attend college "not because they are unqualified, but because they cannot afford it." Counterintuitive though it might seem, a desire to broaden access and increase the number of young Brits in college was exactly why the U.K. opted for the opposite course from the one Sanders is proposing.

Before the introduction of tuition fees, government resources could not keep up with increased demand for university education. Per-student funding dropped by 40 percent in the last two decades of the tuition-free system. In order to keep costs under control, the government spent less per student and rationed the number of spots available. But costs continued to balloon. What's worse, students from better-off backgrounds were more likely to grab one of the limited number of spots, so it was students from poor backgrounds who suffered, with the gap in degree attainment between rich and poor widening.

Britain faced a choice: an elitist university system in which the state subsidized the education of a select few who managed to perform well on high-stakes tests, or giving many more people the opportunity to earn a university degree by shifting part of the financial burden from the state to the individual; it opted for the latter.

Today, students pick up the bill for, on average, around 65 percent of the cost of the education they receive, with taxpayers plugging the gap. More students are enrolled than ever before and those students benefit from more per-student funding than the generations that paid nothing for college.

Democratic presidential hopefuls say they want to get more students from poor backgrounds enrolled in college. In the U.K., the best way to do that was by charging, not by making college free. In the last decade, the number of 18-year-olds from "low participation"––poor––neighborhoods going to college has increased by 75 percent. And over that period of time, funding per student has risen by a quarter. There have never been more young Brits who qualify for free school meals going to college.

As in America, the question of how to fund higher education in Britain is far from settled. Jeremy Corbyn, the leader of the left-wing Labour Party, backed the abolition of tuition fees at the last election. A recent government review of the system proposed a series of modest reforms to address concerns over the current system. One concern centers around the affordability of the current system, given that the government can't know with any certainty how much of the tab it will have to pick up. A report published earlier this year by the London-based conservative think tank Onward forecast that just 17 percent of graduates will fully pay back their loans.

There are persuasive principled objections to taxpayer-funded college. If the individual accrues the lion's share of the benefits of a college degree, why shouldn't he or she bear the cost? Asking taxpayers to fund universal free tuition means asking, for example, a farm worker who earns the minimum wage to pay for the education of a student from a wealthy household en route to a high-flying career in law or finance. You don't need to be an avowed free marketeer to see the problem here; none other than Karl Marx raised a similar objection when he said "If higher education institutions are also free that only means in fact defraying the cost of education of the bourgeoisie from the general tax receipts."

But let's assume that those problems aren't fatal to the case for free college. The problem for the likes of Sanders and Warren is that they cannot escape the affordability dilemma that makes it very hard for free college to live up to its promise. Either other public spending priorities face the squeeze to fund universities, or the number of spots available at publicly funded schools must be capped.

In other words, students paying their own way need not be a barrier to achieving the stated aims of the tuition abolitionists. Sanders and Warren use "free education" and "universal education" interchangeably. The U.K. story is a reminder that that often those aspirations aren't complimentary, but contradictory.