> Does this mean that Farage's party also can't win in the next general election (and make him PM)?

Firstly, that election is three years away; it's a separate political event.

Secondly, that depends on the outcome of at least three hundred constituency elections, which would have to return Reform candidates. They did OK for a minor party at the last council elections, but they have a very high attrition rate as candidates are discovered to be above the acceptable racism threshold, bent, mad, or simply never intended to do the job in the first place.