FTMDiaries - thanks for helping solve the mystery~
Got back from the appointment a little while ago and thought I'd give an update and a walkthrough in case anyone's having one of these appointments in the near future and wants more info/is feeling anxious.
So, the nurse took my blood pressure, height and weight, and worked out my BMI. Then there were the standard "do you smoke?", "how much do you drink?", "do you take recreational drugs?" questions. After that, she stuck a needle in my arm (after complimenting my veins

) and drained most of the blood from my body. Five vials! Five! And they were big vials. She said it was the most she'd ever taken in one go. I'm genuinely concerned that my vampire theory wasn't far from the truth.
That was pretty much it. Really nothing to be anxious about, undead notwithstanding. Took ten or fifteen minutes. Very little pain, retained the vast majority of my dignity.
The blood tests are to screen for "just about everything." Diabetes, deficiencies, hormonal abnormalities. If they do detect hormonal abnormalities, they'll pull you in for further testing which might involve checking whether you're intersex. The nurse was absolutely lovely and very understanding - they all are at that surgery. She asked my preferred name and put a note on my medical records so NHS people know to use it in the future.
I'll sum up by saying it's been a good morning

I'm a centimetre taller than I thought I was! And now I'm off to grow some more blood cells. Thank you all, and a wonderful day to you~