Article explains that National Health Service used many high-risk, often paid, donors. But problem became much worse because it wasn’t one transfusion per donor, but that the collected blood was mixed, so that one infected donor would contaminate blood going to numerous recipients.
Link explains, but only partially. No, the blood wasn’t used in traditional transfusions at all. It was used to make some product they hoped would improve clotting. (A consequence: many of the victims were hemophiliacs.) To manufacture the product they mixed blood from multiple donors.