Tutu today said at the Westminster Abbey celebration that the anti-apartheid campaign in Britain helped keep the movement going.
He said campaigners had helped change the moral climate and prevented Mandela from spending his life in prison and being regarded by many as a terrorist.
Tutu thanked Britons who had picketed South African government buildings in London and led boycotts against the apartheid regime.
Mandela died in December. He had often cited the British judicial system as a model.