@Traci 66
NHSBT have a legal responsibility to screen their donors very carefully prior to accepting them for the blood donation process. They have a duty of care to the donors, to ensure that no harm is caused.
The reason there is a minimum weight restriction is because no donor should lose more than 13% of their blood volume when donating, in order to reduce / prevent the side effects of donating, such as fainting during the donation session, or development of anaemia. The volume of blood donation units is fixed, and, working back, 13% of blood volume translates to a minimum weight of 50Kg, or 7st 12lbs in old money :)