I've given 52 pints in total. It is definitely something you get used to, and have to plan for. Over the years, I've felt faint (but not actually fainted) a few times, and I have to plan what and when to eat before I go.
Do you faint from blood tests because of the sight of the blood? Or the needle? Or the loss of blood? The blood tests I've had haven't involved any significant amounts, certainly not in comparison with the amount they take at a donation.
If it's the sight of blood/needle, I'm with you on that - I never watch, and look at the ceiling/the other side of the room the whole time. I can definitely feel the needle, but just have to block it out.
When you go to donate, they ask you to drink a pint of water/squash when you arrive, which keeps your blood pressure up, and you have another drink after. The seats for donation are sort of like a moulded plastic sun lounger so your feet are raised, and they're on a tilting base, so if you feel faint, they can tilt the whole thing so your feet are much higher. You stay sitting on the seat after the donation until you are ready to get up.
You can speak to someone at the Blood Service on 0300 123 23 23, and I've always found them really helpful when I've phoned with queries about medication (there are rules about donating on prescription medication, but most things are find if you check - I'm on HRT and a couple of other things, and still donate). Or if there's a local donor centre, you could pop in, and talk to the reception staff.