The absence of the United States is not going to turn the clock back to the middle ages. Europe and the world still had other strains of liberal democratic thought, ones more radical than those proposed during the revolution in the 13 Colonies. It's not like they had a monopoly on the ideas after all, contrary to some American myths. Simply put different ideas would emerge, some perhaps more radical and tolerant than those historically. The social pressures would build up eventually as the events of 1848 showed.
It's not going to turn the clock back to the middle ages. It's going to turn the clock back to the 17th century. The Middle Ages died in the centralizing, absolutist impulses of that century and the American Revolution was a reaction to those same impulses in the new century. It discredited and shamed the king's faction in Parliament and the independent United States represented a continuous reminder to the British middle and working class that English speaking men were ruling themselves a mere ocean away through out the 19th century.
The supposition that a successful, independent, democratizing republic laying across the Atlantic from Europe had
no influence whatsoever on European liberalism -- at least on a social scale -- is absolutely ludicrous. Especially, the tea-drinking cousins. Arguments about the evolution of suffrage in OTL 19th century Britain are utterly irrelevant to TTL's 19th century Britain.
EDIT: I mean, this is supposed to be a site for history nerds. Do people not know what the 17th (and 18th, kind of less, because most of the work was done in the 17th) century looked like for 'liberals' (in the loosest possible sense of those advocating for 'liberty', for at least a relatively broad class of people like lower nobility)? Every country you look at, as look deeper, you see the retreat of this 'liberty' in the face of absolutism. The Great Elector and his son crushing the Estates in Prussia. The taming of the aristocracy in France. The liberal existential horror of the Restoration in Britain. The Coup of 1772 in Sweden, the Partitions of Poland, the growth of the fiscal-military state in Britain even after the wonders of the Glorious Revolution, the decline and political ossification of the Dutch Republic, the list goes on and freaking on.
You can debate the real significance of all of this from a modern perspective (read: Not that freaking much. The groaning masses continue to suffer while their task masters bristle at having to experiencing something of the same from their own bigger fish), but THIS is the context that Enlightenment liberalism grew in. THIS is the context that the Commonwealthmen of the early 18th century wrote in, which American colonists lapped up like a dog given steak. This is WHY England/the UK had such a strong liberal streak going into the 18th century: The aristocracy and plutocracy was most successful there at resisting the authoritarian impulses of the monarchy. This is why the American colonists turned to all the same arguments and authors as the Parliamentarians in the Civil War and the Glorious Revolution: They thought the same thing was being done to them.
The American colonists/United States operated in a common ideological context with liberals in Europe. What happened in America had repercussions in Europe and what happened in Europe had repercussions in America.
The space of alternate histories is vast and our human minds are limited, but I think it's pretty fair to say that, in many possible histories, a world where the American Revolution fails is NOT a world where Europe liberalizes at the same speed or to the same extent as ours. I mean, imagine if you lived in a timeline where the French Revolution succeeded at establishing a stable, liberal French Republic that survived with full continuity down to the modern day. Would you argue that the events of our timeline, where Reaction set in and prevented democracy anywhere in Europe for half a century wasn't possible?
EDIT2: OK, so just look as close as you can at the example of the Riksdag. Sweden in 1770 looked surprisingly like the UK on a political institutional level. There was a greater degree of partisan chaos, but the king was
mostly powerless and the parliament ruled the country with a relatively similar level of unquestioned authority. The mere fact that Sweden did relatively badly in a single war was enough to entirely reverse a half century of parliamentary rule and replace it with a half century of more-nearly absolutist monarchy.
While trends and historical directions certainly exist, prior to the American Revolution the trend and historical direction was distinctly illiberal.