Ok, preparing to get shot down for this, but what the heck
The fact she has gone downhill rapidly overnight and is now refusing her favourite foods is not a good sign.
If she worsens any more, do the decent thing and get her to a vet to be PTS.
Would you want to be left to die in pain? No. Is it worth the stress to take her to Northampton to the vet? No
I have done this, this is exactly how my beautiful baby girl Whisks left us last October (24th, 8.45am, will never forget it). My vet, who does know a lot about guineas, talked it over with me and we decided that it wasn't worth her struggling on for days/weeks etc in this unfair quality of life. She fell asleep peacefully and I carried her home in my arms.
When we take on an animal we sign an invisible contract to do the best we can for this animal, and to have the guts to make the hard decisions when the time comes. It is incredibly hard to know when to say goodbye, but at the end of the day, you have to listen to what your animal is telling you, not your own emotions of 'oh but I could hold her one more time' etc - it just isn't fair.
I am a lot harder than this than many on here, maybe because of my own experiences, but as I say I have been through the exact same and made that decision and I know in my heart my girl went before it got too painful for her. I personally could never hang on to an animal if I knew it was in that much stress.