Tony Benn was Postmaster General of the U.K. from 1964 to 1966. At the time he was known by his formal name, Anthony Wedgwood Benn. One of his primary efforts as PMG was to modernize stamp design (in part to increase postal revenue). As a republican, he believed that an important part of this modernization was to remove the Queen's portrait from stamps.
(In the U.K., a republican is someone who wants to abolish the monarchy and replace it with a republic that has a non-hereditary head of state.)
He commissioned David Gentleman to create stamp designs that utilized symbols such as a coat of arms to replace the monarch's portrait. The image above shows one of Gentleman's designs. Benn showed the designs to the Queen, as well as others, but as we know, he was ultimately unsuccessful in his quest. He did, however, win a compromise that the portrait could be reduced to a small cameo.
In early 1965, Benn met with Sir Kenneth Clark, chairman of the Stamp Advisory Committee, to tell him about his ideas for new commemorative stamps. Clark took the opportunity to suggest a new profile version of the Queen's head for definitive stamps; the committee and others were dissatisfied with the design of the Wilding series with its three-quarter photographic portrait of the Queen.
Benn agreed, and that started the effort which led to the Machins over two years later. By that time, Benn had moved on to a new position in the government, so he is not generally associated with the Machins. However, it was clearly his efforts that paved the way for radical changes in design of both definitives and commemoratives.
British Postal Museum & Archive and is highly recommended for any serious Machin collector or anyone interested in the history of modern British stamps.