Can you not go and see the GP you have usually seen?
I went to see my GP, and it was a completely new practice for me, so i had never met her. I had a good chat with her and she looked through my history asked me questions and happily said that as long as on the day I am fine she sees me as medically fit for surgery.
I have crohns disease and Im on a treatment so also had to go to my consultant and get a letter from him too, my GP also put in the letter as long as the consultant sees fit too.
They dont have to agree with the fact your having a BA they just need to write facts. both of mine said ‘ as long as shes sympton free around the time of surgery’…
Your gp is most likely scared of taking responsibility just incase anything did happen. but they can word their letter in many different ways .. you should definitely go and chat with them instead asap! good luck xxxx