In which case, here are some special people I have come across in my studies in psychology:
John Bowlby - Wikipedia -- he really revolutionised how we view child development. Many people have heard of Harry Harlow's monkey experiments into attachment theory but Bowlby got there first and with considerably more compassion and rigour. He inspired worldwide a new way of looking at how we should bring up children. Before Bowlby, the attitude was ignore babies lest they be spoilt. After Bowlby, we realised that this was a good way of rearing psychopaths.
Susan Sutherland Isaacs - Wikipedia -- another titan of child development. She realised that independent play was a crucial part of learning how to deal with the world. Like Bowlby, Isaacs truly changed the world.
Melanie Klein - Wikipedia -- took psychoanalysis into the realm of children, thus opening up the idea that a child's world is distinct from that of an adult and crucial to study in its own right.
Donald Broadbent - Wikipedia -- an astounding thinker who pretty much single-handedly invented the study of attention (alright, that's hyperbole -- all scientists build on other work, but Broadbent's innovations were myriad and brilliant). As if that wasn't enough, he leapt from that to invent whole new areas of cognitive psychology.
Anne Treisman - Wikipedia -- whilst we're on the subject of attention, Treisman needs a mention. We owe a lot of what we know to how human beings filter and select items for attention (and ignore others) to Anne Treisman, who sadly died just this year.
All brilliant British scientists of the 21st century (and beyond). If I had to choose one -- which is tough -- I'd probably go for Bowlby, because he really did silently change the world in a really fundamental way. But any of them would grace the £50.