A journey of a thousand miles that began with a single episode ends tonight with an 11th. "Brideshead Revisited" concludes with a 90-minute chapter, at 8 on Channel 26, that takes place in the winter of 1939, when Lord Marchmain (Laurence Olivier) comes home from France to die.

Unfortunately for "Brideshead" and its small but intensely loyal legion of followers, the demise of Lord Marchmain and the series occurs on a day abnormally overstocked with momentous television. In fact, David Brinkley reported yesterday on ABC's "This Week with him " that President Reagan had decided to postpone a press conference planned for today because he didn't want to compete for television attention with the Oscars, the NCAA championship and the landing of the space shuttle Columbia.

It will be possible to watch all of "Brideshead" and see most of the annual Academy Awards telecast, at 9 on Channel 7, but many homes will face an irreconcilable conflict between "Brideshead" and the NCAA finals in which Georgetown University is a competitor, at 8 on Channel 9. The simple lesson of this is that no American dwelling in which more than one person resides should try to subsist on fewer than two working television sets (not counting the one in the fallout shelter, of course).

For the true "Brideshead" loyalist, the choice will not be so difficult. Having come this far with Charles, and Julia, and that besotted sprite Sebastian (gone, but hardly forgotten), one feels obliged and privileged to see things through. It's not a matter of just learning what becomes of Charles Ryder (Jeremy Irons), because we know little will become of him. "I'm homeless, childless, middle-aged and loveless," he announces near the journey's end. But he is also the quintessential outsider, and this gives him a certain doomed pathos.

Those who have remained faithful to the series will find this a fittingly rewarding and cathartically traumatic conclusion. Lord Marchmain's condition worsens until, says Charles in his narration, "presently there were no good spells, only brief fluctuations in the speed of his decline." The central question is whether Marchmain will consent to receive the last rites from a nearby parish priest. At the first attempt, he sends the priest away, but the priest remarks, "I've known worse cases to make beautiful deaths."

One of the longest flashbacks ever recorded on film concludes, and the series ends almost exactly the way Evelyn Waugh ended the novel on which it is based.

The surface glories of "Brideshead" are obvious and have been adequately celebrated. But they are not the principal reason for the impact or popularity of the program. A frump or a heretic here and there has tried to gain a little notoriety for himself by scoffing at "Brideshead" in print; these people think themselves courageous in standing on the sidelines and jeering that the empress has no clothes. But she does, and what clothes, and how sad not that some people don't appreciate "Brideshead"--for that is their right--but that a supercilious few choose to interpret this attitude as a sign of shrewd sophistication.

Among the foolish charges made are that the popularity of "Brideshead" can be attributed purely to Anglophilia and that the program is nothing but a fancy-dress soap opera. Both allegations are ridiculous. A passion for British telly may have explained the success here of "Upstairs, Downstairs" and many of the "Masterpiece Theater" pomposities, but "Brideshead" is not about England or the English to any limiting degree. It's about so much more.

And there's also much more to it than a soap-operatic what-happens-next. Purely as a plot, it meanders, or makes sudden leaps over unexplored chasms. These are not the adventures of Charles Ryder; Charles is too passive, for one thing, and at times he is even a bit of a cad (although he is always sympathetic as played by Irons, in a vaguely victimized style that recalls Leslie Howard, especially in "Of Human Bondage").

On its most general level, "Brideshead" is about remembering anything and being unable to recapture it--a lament for lost grace, a graphic confirmation of our suspicion that beauty gets a little less plentiful in the world with the ascent of each succeeding generation. A devotee of the program will find it hard to comprehend how some people are unable to see and appreciate not only the elegance of it but also the depth of it--the charm of it, the power of it, the Catholicism of it, the wisdom of it.

The word "nostalgia" has taken on such a cloying, superficial meaning through overuse that you wouldn't want to apply it to "Brideshead," except that the program does evoke, sensually and spiritually, the particular mood of the world between wars, and also of a time when--or so it now seems--people still controlled technology, rather than the other way around.

Not the greatest TV program ever made? Slow-moving and dry? These things can be said of "Brideshead," but they're irrelevant. On television, it's not what things are but what they represent. "Brideshead" for many goes beyond esthetic considerations; it's one of those rare works that insinuates itself into memory alongside primary experiences. In the future, it will jiggle around in the memory bank like a first date or a lost love or one particular night of good company and laughter. Or one particular morning of rueful realization.

There are so few basic pleasures in life, really. Sex, money, chocolate and "Brideshead Revisited." It's partly a tribute to the mercurial allure of the program that, finally, its appeal and its magical spell cannot be fully explained by anybody. Very soon it will be something to look back on, and something to hope public TV might repeat annually, or once every five years, or eventually at least, though the first time will always have been the best. At least it meets a beautiful death tonight.

Farewell to Brideshead, farewell to the chapel, farewell to Oxford, and Cointreau, and Venice, and Aloysius, and Mr. Samgrass, and the monogrammed turtle, and "such a lot of nonsense," and wine and strawberries under a tree, and innocence and youth and duty and honor, and to all we hoped we one day would become.