Just done a quick calculation on vote share versus seats won, and translated that into how many seats each major party would've won under 'pure' PR (i.e., percentage vote = number of seats).
FPTP
Labour 411
Con 121
Reform 4
LibDem 71
PR
Labour 219
Con 154
Reform 93
LD 79
So under PR you'd maybe have the right-wing of the Tory party collaborating with Reform and the majority Labour government having to rely on the LibDems for support. As we saw in 2010, when the LibDems made a huge mistake in committing to a coalition, that doesn't make for good government.
The other what-if factor was that Reform was second in 9i constituencies. I might do the maths later for individual seats but let's assume that in 74 of those the Reform vote made the difference between the Tories losing and winning, and that 50 of those went to Labour, and 24 to Lib Dems. Assuming negligible Reform support, or no Reform at all, that would have made the final result (bar the 2 missing results):
Labour 326
Con 196
Lib Dem 53
Corrected - So the barest Labour overall majority of one.