President Carter named a leading consumer advocate to an Agriculture Department post yesterday and appointed a personal representative to participate in planning sessions for a possible mid-May economic summit conference in London.

Carol Tucker Foreman, 38, executive director of the Consumer Federation of America, was named assistant secretary of agriculture for food and consumer services.

A native of Little Rock, Foreman is a former director of research and publications for the Democratic National Committee. Prior to joining the consumer federation she was executive director of the Citizens' Committee on Population and the American Future.

As his representative at talks to plan the economic summit, the President chose Henry Owen, 56, a former State Department official and director of foreign policy studies at the Brookings Institution.

The economic summit would be the third such session among leaders of the United States and the industrial nations of Western Europe and Japan. It is likely to be held in connection with a mid-May meeting in London of leaders of nations belonging to the North Atlantic Treaty Organization.

Carter is expected to attend both meetings in his first tip abroad as President.

In other announcements yesterday, White House press secretary Jody Powell said the President will nominate Patricia M. Derian, a leader of Democratic reform politics in Mississippi, to be the State Department's coordinator for humanitarian affairs, and James Gustave Speth, a cofounder of the Natural Resources Defense Council, to be a member of the Council on Environmental Quality.

Powell also announced that the President will hold his third press conference on Wednesday, probably at 10 a.m.