Farage can't realistically be PM in the current parliamentary session anyway. His party needs to win, or do very well in, the next general election instead. At a national level this election is purely symbolic and dictates whether Reform will have 7 or 8 seats out of the 650.

I suppose that in theory if Reform could form a government after the next election, but Farage still didn't get a seat, a party colleague could become PM and appoint him as a peer. There's a rich history of politicians losing elections and getting appointed this way instead, though as far as I know none of them subsequently became PM.