It's time to see if the polls are right.
Previously: the voting megathread
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Direct proportional representation is easy enough. Just look at the number of votes each party got, and assign that percentage to their overall parliamentary representation. That roughly gives you the answer.
IRV is more interesting, but more complicated. It relies on some assumptions (e.g. Green, SNP, LibDem, Labour all preference each other 100%, Conservatives & Reform preference each other 100%) and takes a lot of effort to do on a seat-by-seat basis. And of course it all assumes ceteris paribus, when in actuality people would vote differently if the voting system were different.
As one example, here's the seat of Tatton:
Under IRV, with the above assumptions, Labour's Ryan Jude would have won with 26,005 votes to Conservative Esther McVey's 25,904. But tweak those assumptions just slightly (give 90% of LibDem votes to Labour, 10% to Conservatives) and it could go the other way (26,365 CON to 25,544 LAB). There are dozens or scores of seats where these sorts of interesting hypotheticals can be asked and analysed. IRV is actually, in my opinion, the next-worst voting system after FPTP (if you exclude weird and rarely-used ones like approval voting, range voting, etc.), but it's one of the most interesting to do analysis with.
STV is an utterly impossible comparison to make, because it relies on multi-seat electorates, which would probably be done by merging existing electorates into groups of 3–8. STV is a more generalised case of IRV so if you decided on how to do those merges, then you can get even more interesting analysis. As one example, if we imagine a merged electorate involving West Ham, East Ham, Ilford North, Ilford South, Leyton & Wanstead, and Stratford & Bow. Some assumptions are necessary to make this work, my assumption is that anyone whose party name says "workers" or "socialist" preferences Green and then Labour, while those mentioning religion preference Conservative, and if I don't know, I'm giving them to LibDems then Labour. I'm also assuming all voters for a named party vote as a block, preferencing the same candidate 1st, 2nd, etc., while independents get the votes as they were actually given. This is somewhat realistic because ballot paper design can be set up to encourage this in an STV context (see how Australia does it with "above the line" voting in the Senate, for an example). I've merged the minor parties named "workers" or "socialist" into a single party.
A detailed explanation of my calculation is contained here.
In our merged hypothetical under STV, they win 3.03 quotas on first preference, Conservative wins 0.88, Reform 0.41, LibDem 0.28, Green 0.78. So Labour immediately win 3 seats, before 0.03 quotas are distributed lower down in their party. After numerous more rounds (my attached spreadsheet simplifies multiple rounds that by eye would obviously not result in a new quota being reached being merged down into 1), the LibDems win a quota. I've decided to distribute their excess 50/50 to Green and Conservative, since Labour has already been eliminated. To be frank, after that I'm not sure what to do. LibDems having been eliminated, the 3 remaining independents can't go to them as was my initial plan (the basic thinking being independents are probably more centrist, but LibDems and Labour being eliminated already. I've decided to give them 50/50 to Conservative and Green, but the reality could be so, so much more complex. After one of those is eliminated, Conservatives get a quota. One final independent distributed to Conservatives/Green and the Greens win the last quota.This is Labour heartland and in FPTP Labour won all 6 of these seats. My calculation ends up with 3 Labour, 1 LibDem, 1 Conservative, and 1 Green.
MMP ends up with basically the same overall result as direct proportional, but can be interesting in terms of independents & very minor parties and resulting overhang seats.