A novel on behalf of lonely, middle-aged professional women; Foreign Affairs, by Alison Lurie. New York: Random House. 291 pp. $15.95.

About two-thirds of the way through Alison Lurie's seventh and latest novel, ''Foreign Affairs,'' Virginia Miner, the 54-year-old protagonist, asks herself: ''Why, after all, should (she) become a minor character in her own life? Why shouldn't she imagine herself as an explorer standing on the edge of some landscape as yet unmapped by literature: interested, even excited - ready to be surprised?''

''Vinnie,'' a not-entirely-likable American academic, is in London. Actually , the reasons that she shouldn't imagine herself in this manner are many, but the fact is, she has fallen in love - against her better judgment but with the approval of heart and body. The object of her affection is one Chuck Mumpson, an unsophisticated, rough-cut but amiable retired sanitation engineer on a tour of England.

Vinnie is plain. She has been married once - briefly and unsuccessfully. She feels sorry for herself too often. She is, to state the stereotype ''Foreign Affairs'' works to explode, a spinster professor.

Vinnie has come to London on a grant and so has another professor from the English department at Corinth University in upstate New York, where Vinnie teaches. This is Fred Turner, an assistant professor studying John Gay (''The Beggar's Opera''), and just recently separated from his wife, Roo.

Fred is quite the opposite of Vinnie - handsome, outgoing. People stare at him trying to figure out in which movie or television show they saw him. ''Fred is not embarrassed by this attention,'' writes Lurie, ''he is used to it.'' On top of the good looks, he has always been an ''all-around achiever.''

Fred drifts into an affair with a well-known English actress, Rosemary Radley. Beautiful and feminine (in nonfeminist fashion), she has Fred jumping through hoops. When he finally sees past her elegant surfaces, though, what he finds is a life in schizophrenic shambles.

''Foreign Affairs'' shifts in point of view back and forth from Vinnie and Chuck to Fred and Rosemary, and the reason for this soon becomes only too obvious: Lurie wants to prove to us that the plain ones find love, but the lovely ones come up empty. Of course!

Lurie's novel, aside from probing the differences between appearance and reality, seems largely a brief on behalf of Vinnie, as symbolic of lonely, middle-aged professional women whose lives, Lurie wants us to know, are considerably fuller, or at least have more potential, than we imagine.

True enough. The problem is that in ''Foreign Affairs'' Lurie does too much telling and not enough showing: The soapbox from which she voices her opinions is sometimes visible, as are the seams that connect this well-written but basically conventional novel.