Worth bearing in mind that the UK has just switched to fixed five year terms for elections. (Although whether it will work in practice is in question.)
It should also be remembered that a parliamentary system isn't necessarily proportionate, so there isn't necessarily the super strong drive to multiple parties. A first-past-the-post system tends to mean you get a drive towards a two party system within each constituency. The fact the US has a national two party system is because the President as a national candidate. I think a parliamentary system is much more likely to cause a drive towards geographic parties: I can imagine the federalists lasting as a Northeastern party for example.
It should also be remembered that a parliamentary system isn't necessarily proportionate, so there isn't necessarily the super strong drive to multiple parties. A first-past-the-post system tends to mean you get a drive towards a two party system within each constituency. The fact the US has a national two party system is because the President as a national candidate. I think a parliamentary system is much more likely to cause a drive towards geographic parties: I can imagine the federalists lasting as a Northeastern party for example.