Well done Magwitch I have given blood many time now to the NHS Blood and Transplant (England) each time I'm so pleased that I could be helping someone else.
You have to fill in a In depth questionnaire and have a short interview with a nurse before you can donate, some questions are very personal. On two occasions I have not been allowed to donate, once due to travelling and taking anti malaria tablets, six months grace was enforced, I was honest which all should be, if you are not sure about anything talk it out with the nurse on hand.
The safety of the blood supply is really important and is taken seriously. Only 4% donate blood here, so plenty more could.
I normally feel OK after donating but the last time December I did feel a little weak, not sure why as I usually feel very good. The proof: http://www.flickr.com/photos/whosphoto/4109519106/
frogit