The 50 portraits, whose subjects include Sir Lawrence Oliver, Sir John Gielgud, Sir Alec Guinness, Henry Moore, Lord Mountbatten and Harold Pinter, were exhibited last month at the Light Gallery on Fifth Avenue, and have just opened in London. Meanwhile, the book version of the prints, with extensive commentary, has been published this month as _The Great British_ (New York Graphic Society, Boston, $14.95). The photographs, like those found in Newman's three previous books and in his hundreds of assignments for _Life_, _Look_, _Newsweek_ and other publications, are far more than mere portraits. Rather, they are profound artistic statements, in which the background of the picture often symbolizes the person's achievement. "I don't use props: I use reality," explains Newman, taking a break at the West 57th Street studio he has occupied since 1948. On the wall are pictures -- he prefers that word to "photographs" -- of Marc Chagall, Pablo Picasso, Eugene O'Neill and four American presidents; Newman has photographed every president since Truman. Big, burly, mellow-voiced and casually dressed, Arnold Newman at 61 looks like an aging beatnik. His quick wit and ready laugh mask a perfectionism that has characterized his work ever since he turned to photography in 1938. His ability "to make the camera see what I saw" showed itself almost at once. In 1941 he held his first exhibition and sold his first print to the Museum of Modern Art. "I could have made, over the years, a hell of a lot more money than I have, simply by doing more commercial work and cashing in on my reputation. But that doesn't interest me," he reflects, puffing on his ever present cigar. "I mean, money interests me, but I'd just see my life being wasted." Specializing in portraits of artists, he studies the work of each subject intensely beforehand so that the essence of the artist will be distilled into the photograph, by subconscious as well as conscious effort. On the side, he does enough commercial work to support his own artistic efforts. But over the years, the two have somehow merged: "I'm forever being commissioned for things I'd give my eye teeth to do, and paid very well for it. Recently I went out to do a photograph strictly on my own of somebody I admired, and I hate the picture. Yet the day before I did a picture for money which I think is one of my best pictures in the last three years." In 1953, he went to Washington to photograph 15 U.S. senators for _Holiday_ magazine, including John F. Kennedy -- then a political unknown who was sometimes labeled the Playboy senator. "Years later," recalls Newman, "I was photographing President Kennedy on the White House lawn. He turned to me and said, 'Arnold, whatever happened to that first picture you took of me?' "I said, 'Well, Mr. President, we did 15 senators, and they found out they had one too many for the layout, so they dropped the one least likely to succeed.'