So sorry to hear of your loss, Remy

Sheep can and do drop down dead overnight; we had one just this week. They do say of sheep, often the first indication you have of any illness is a carcase. They're very good at masking symptoms, even when you're watching them closely they can appear symptomless one day and dead the next.
If she was killed rather than died, then I'd say it's far more likely to be badgers than foxes; generally I'd say foxes can't manage healthy adult or near-adult sheep and don't try.
The fleece and remnants that you found - was any of the fleece/skin turned inside out? If so, that's a sure sign of badgers.
Otherwise I'd suspect dogs sooner than I'd suspect foxes. Cubs will be hunting on their own now; I suppose there's an outside chance some siblings have stayed teamed up, but you don't hear about it. Foxes will, of course, clear up any dead or disabled sheep in short order, however.
Are you up to date with fluking, minerals, vaccinations? If you run the sheep around the field for a while, are any of them noticeably more out of breath / labouring to breathe than the others? It's been a horrible year for stock, they are all struggling more at this time than they would normally be, so any ailments will take a greater toll than they would in a more normal year.