In this case, they likely had adequate fuel for, the usual eventualities but the weather in Scotland was particularly bad that night across the whole country (source: I live near Prestwick airport).

Either Edinburgh (on the east coast) or Prestwick (on the west coast) are ok (one or the other or both) but in this case neither was suitable so the nearest was Manchester - definitely an edge-case.

I don't know how much fuel they had, or if they could've fitted any more on the plane but it was unusual circumstances.

There was a military plane right behind it with the same issue that night too.