I gave blood twice, both many years ago.
The first was when the came to my then local community centre. I was told to roll a cylindrical object in my hand to aid blood flow before, almost immediately, being told to stop that and lie very still. My ex wife started her donation several minutes before me and was still there when I had completed. I passed out for a short while. The team put it down to me losing the blood too fast and suggested, the next time, to go to one of their centres. I’m not squeamish at all so was able to discount that.
Second time was at the centre in Glasgow and I explained the issue from the previous time. They took good care of me and monitored me throughout. Again the donation started off too quick to the partially clamped the line. Again I was faint at the end and was advised, as I have a common type, it was not in my own best interests to continue donating.
I’ve recently applied for a job with SNBTS ( Scottish National Blood Transfusion Service). If successful I may revisit the idea.