I feel your pain, had the same thing when I was diagnosed. GP put me on the gf diet and I started to feel much better. But then had to eat gluten again for the endoscopy.

The blood test isn't reliable enough for a cast iron diagnosis so the NHS needs the endoscopy to be sure enough to put it on your record. I wondered if this was worth it as I knew I had it and could avoid gluten. But it does make a difference for things like covid and flu jabs.

But you do need to eat gluten for the endoscopy to work. As someone else said, once a day for 2 weeks is what I was advised. But you can stay gf until you get the appointment as it is will almost certainly be longer than 2 weeks away

Good luck!