The first half of the quotation is from George Bernard Shaw, as reported by his biographer Hesketh Pearson. The second half is from Pearson himself.
On the evening of October 10th  my wife and I called to see him [Shaw]. Whitehall Court had been buzzing with reporters, two of whom had taken a flashlight photo of him, but the excitement Was now subsiding, and we enjoyed a tranquil talk. He was lying in bed, looking better than I had seen him for years; his complexion was rosy, like that of a child; he seemed to be thoroughly enjoying the effects of his accident; and he talked with vigour, though the lower set of his dentures was on the table beside him. The crutches with which he had hobbled about during his breakdown in 1898 had been discovered, and were propped against the wall by his bed. The object of my visit was to find out all he could tell me about Keir Hardie, a film of whose life I had been asked to write; but before we left we had heard his views, not only on Hardie, but on Sir Edward Grey, Ramsay MacDonald, Napoleon, Wellington, Field-Marshal Montgomery, T. E. Lawrence and Tom Paine. The English, he said, were a nation of amateurs, not professionals: their generals, like their authors, were amateurs. “Which explains why we have always won our wars and have produced the greatest literature in the world,” I remarked.
Hesketh Pearson (1951). G.B.S.: A Postscript, p. 130. London: Collins.
“Nation of amateurs” is not original to Shaw—it was first said by Archibald Primrose in 1900, and by 1946 was a familiar expression.