With the addition of more parties to the German political spectrum, building coalitions has been harder in recent years, forcing essentially the two most central mainstream parties, CDU/CSU and SPD, to build a "great coalition". The problem of this is that people not happy with the results necessarily have to vote more extreme, and we did see a lot of protest votes, swinging from the very-left Die Linke to the ultra-right Afd, for example.

Observing the political process in the US, it seems that the primaries leave the final candidates always somewhat worse for wear, maybe more so on the Democrat side than on the Republican side (Infighting seems to be a worldwide phenomenon more prevalent on the left). By necessity, most winning candidates must then do a dance from the more extreme positions they expressed to win the primaries to the center. This is almost by construction, as the voters in the primaries will almost always be more extreme than the voters in the presidential election, even if reduced to those which would consider to vote for the party in the first place.

The basis of a two-party system is actually single-member electorates. Because each seat can only have one representative and the economics of campaigning and organising make it hard for a third party to make appreciable inroads.

Australia's Parliament demonstrates the differences: the House of Representatives is composed of single-member electorates and is dominated by the two major political organisations (and this is for the good, as the government is formed in the House and a two-party system almost guarantees that a government can be formed and sustained easily after each election). The Senate, by contrast, is composed of multi-member electorates, one for each state, elected on a proportional basis. While the major parties typically have the largest share between them, the nature of a proportional multi-member electorate is that it creates more room for minor parties to obtain one or two seats.

We also have a parliamentary system, where these parties can make coalitions and essentially pool votes, which softens (but doesn't eliminate) the need for strategic voting.

POTUS, on the other hand, is personally entrusted with substantial levels of power.

The constituent countries each have a nationalist bloc that will often be successful in local government but not really at the UK level unless whoever is running the UK as a whole is antagonistic to the nationalist position. This results in silliness like Wrexham (which is technically over the border into Wales but nobody there speaks the Welsh language) being required to have Welsh-speaking bureaucrats and signs in Welsh even though the most likely language you'd hear there besides English is... Polish. Why do this? A few million quid on symbolic gestures defuses Welsh nationalism, cheap at twice the price.

But those nationalists won't work together and have almost nothing to say about the UK's larger politics. The closest is probably either the Scottish Nationals objecting to the consequences of Brexit (since Scotland didn't vote for it, and it destroys at least one of the claimed benefits of voting against independence for Scotland in its previous referrendum) or the DUP (a Northern Irish party which opposes Irish independence and is closely affiliated with the Conservative Party) offering a Confidence & Supply deal to keep the Tories in power.

Beyond that, there's the Liberal Democrats, but they aren't very big, and the only time Brits gave enough Lib Dem MPs the nod, they were obliged to throw their lot in with the Tories†, who aren't stupid and used this to ensure most of the blame for the resulting Tory policies landed on Lib Dem MPs who promptly lost in droves. They are nowhere close to being a "viable party" in the sense of able to govern, though they do have majorities in some local governments.

And then what, one Green MP? I mean, she's competent, her policies make sense, all credit to her, but on her own she is not a "viable party" in that sense. Her constituents (Brighton) are going to vote for her because they trend hippier than most of the UK (also gayer, but I don't think the Green Party is especially pro-LGBTQ), because she's good at her job and incumbents are always harder to defeat but that doesn't result in a Green UK government, just one MP.

So no, really there are two viable parties, the Conservative and Unionist Party (Conservatives, "Tories") or the Labour Party. I think they're both a bit screwed because they both subscribe to beliefs about the importance of Work and I believe we're at the end of the era in which the capability of humans to do Work is necessary or perhaps even relevant to prosperity.

† The existence of numerous smaller groups some of which I did not mention means that the Liberal Democrats could not have formed a working coalition with Labour, because that coalition would lack the votes to beat a "No Confidence" motion from Tories and other rival parties, forcing fresh elections. With the Conservatives they had more than enough, so the only alternative to a Tory coalition was fresh elections. In hindsight that would have been the correct choice perhaps, but wisdom at the time was that the one thing voters hated more than the other party was elections, so never force fresh elections unless you're confident you can win them.