With PR, they would be the largest party.
However that wouldn't neccessarily be a problem.
According to NowCast, Reform would win 171 seats. To form a majority government they need 326 seats.
So, they go cap in hand to the Tories - but they only have 119 seats. That's 290 seats. They still can't enact any stupid legislation.
In the meantime, Labour (122), Lib Dem (82) and Green Party (109) could form a coalition with 313 seats and hope that more moderate conservatives would be inclined to side with them or approach the SNP for a full rainbow coalition with a majority.
The problem with Reform is that even the Tories may not want to work with them, and the other parties definitely wouldn't.