I had a bad experience last year.
There were plenty of appointments available near me, even at weekends which surprised me. The reception staff were lovely and welcoming but the nurse or whoever it was who was reviewing my forms and asking me questions was awful! So rude! She treated me like I shouldn't be there, as though I was asking her for a kidney, not trying to donate my own blood!
My appointment was at 12pm and by then I'd had just over one litre of water that day, I'd had a small glass of orange juice with breakfast and about half of the pint of water I was given to drink in the waiting room. She scolded me for not drinking enough water. I apologised and asked how much water I should have drank by 12pm, she wouldn't tell me, just kept saying I hadn't had anywhere near enough water. I explained I wasn't told how much water I needed to drink when I booked the appointment. She said I can't donate when I'm dehydrated (I bloody wasn't dehydrated!) She stood up and directed me out of the little room we were in. I asked how much water I'd need to drink roughly so I'd know for next time, she wouldn't tell me, just said plenty. She had such a bad attitude.
I made a complaint when I got home. Such a waste of my time. There's no reason why they can't provide some kind of guidance on how much water to drink when booking the appointment. What's with the secrecy? What the hell is "plenty" supposed to mean? I thought I'd drank quite a lot that day for someone who is only short and slim but I'd have drank more if I'd known - but how much more? I'm not Psychic Sally!
That's the longwinded way of saying it's really put me off going again. I used to donate years ago and it was a really positive atmosphere, I never had anyone telling me off or being rude to me.