One of the almost immutable laws of politics is Duverger's Law which says that single member constituencies tend towards two party politics. The reason for this is rather obvious if you ponder it for more than a minute. Put simply, there are only two states that a politician can find themselves in: in or out. A politician who is in, has a seat and wants to retain it and a politician who is out, does not have a seat and wants to attain it. Multiply this by a few hundred times and with single member constituencies, you've just built yourself a parliament which is subject to those exact same conditions. There are only two states that a party can find themselves in: in or out. When this experiment is repeated several times over through the process of many election cycles, then forces tend to coagulate until there is a major shift in the entire political climate.
In Britain, the first two major political parties were the Tories and the Liberals. This only changed after the massive shift in eligibility of the franchise, when working class men and then finally women got the vote. The Liberal Democrats were the big losers out of this and the Labour Party replaced them as the other major party opposed to the Tories. They made some inroads in 2010 but after they formed a coalition with the Tories and then proceeded to show a complete lack of any spine whatsoever, they effectively committed political suicide and will probably only be relevant again in the event of hung parliaments and they end up being king makers.
2017 poses an interesting problem. The Tories are expected to wipe the floor again and form government in their own right again but given the recent events in Manchester, the polls have tightened considerably and the possibility of a hung parliament with the Tories falling marginally short of winning the necessary 325 of 650 seats, the question of who can actually get the required number of seats on the floor of the House Of Commons is again pulled into sharp focus.
If we assume that the Tories get say 317 seats, Labour gets 235, the SNP 59 and the rabble of Northern Ireland accounts for 24, then that only leaves 15 seats in the squabble for the parties to court. Labour and the SNP together only make 294 seats and if by some massive insane turn of events a "everyone hate the Tories" coalition is formed, then we have to consider the daunting prospect of opening the Schrodinger-Pandora Box that is Northern Ireland and negotiating with the rabble.
Of course the SNP could form a coalition with the Tories and provided they show more spine than the Liberal Democrats did, then that's the end of it. The clock is then reset and an election like 2015 would play out in 2022 except with the SNP in place of the Tories. The thing to remember here though is that the SNP have done a fairly successful job of planting themselves in the place of the Tories in domestic politics north of the border and I'm not sure if they really want form any coalition with the Tories. This seems to me like a marriage of inconvenience and if that were to happen, then Jeremy Corbyn could very well be the next Prime Minister by virtue of not really saying anything, after 2022.
We could also face the very real prospect of strong and stable leadership by total absence of anyone at the wheel. If we assume that the Tories fall short of being able to form Government and literally nobody wants to join them, then a situation as happened in Belgium might emerge where there was no Government at all for more than 500 days. Admittedly by all accounts the country continued to bumble along as normal, with the civil service merrily doing its job without the interference of politics; so that might not be a bad thing. Or it might be a very bad thing if you consider that someone probably needs to be around during the negotiations with the EU surrounding Brexit. If that were to happen then precedent exists and the Crown would probably appoint the Tories as the Government as we the case in the 1920s when the Governor General forcibly held a government to remain in power during the King-Byng Affair. Although this is a slightly different circumstance to both the King-Byng Affair in Canada and The Dismissal in Australia, both prove that the Crown and by extension the monarch, has the power to both appoint and remove governments if necessary and Britain which has no written Constitution would look to those examples and be perfectly fine with doing something similar because noise and confusion is preferable to actual anarchy.
All of this assumes that the Tories are going to fall short of the required number of seats needed to form government. All of this collapses if they get to 326 seats because then a government is formed and Duverger's Law again holds because you will have one party in, and all the others out. When 2022 rolls around, it will be all on again.