LONDON (AP) _ Prime Minister Margaret Thatcher of Britain said Saturday she was sending President Reagan a second get-well message before his operation to remove a growth on his upper intestine.

''We have sent a message wishing him well, and we are going to send another,'' she told reporters at a reception for a children's group touring Europe. ''I feel sure he will come through. He has tremendous strength.''

Mrs. Thatcher, the president's closest European ally, said Reagan's wife, Nancy, ''is marvelous and she will pull him through with the help of the doctors.''