Normally if you are connecting onward from a hub airport with a First ticket you can use the First lounge from your starting point if your first flight only has Business seats.

And since Manchester is so terrible that you can hardly fly anywhere useful directly, lots of people are doing this.

Sure, that’s true, but I can’t think of First class lounges anywhere that the provider isn’t providing F seats from. I mean it’s pretty rare to have a dedicated F lounge outside of a hub airport in general for a carrier. QR — who are known for their luxury — only have an F lounge in Doha.