"Upstairs, Downstairs" is going out with the same style, that it has displayed on our Public Broadcasting Service stations for the past four years: High Tea at the Ritz Carlton Hotel in Boston on the eve of the last program. The full cast of characters will be on hand after the last episode tonight to recall good times and raise money for PBS. A final celebration will be hosted Monday by the Mobil Oil Corp. at Sardi's in New York.

"Upstairs for cocktails, Downstairs for dinner," read the invitation from Mobil. It's an invitation that one would like to accept, but with reservations. Where would one belong? Where would one find the proper place - above the salt, below the salt or in my lady's chamber? If you asked the butler, Hudson (Gordon Jackson), would he turn to Vince Sardi and ask why tradesmen did not use the rear entrance?

Let's be honest with ourselves. We will miss "Upstairs, Downstairs" because it allowed us to bring into the open our closet fascination with the idea of class.

This was made quite simple for us in the case of "Upstairs, Downstairs"because the idea of class and the dramatic complications and opportunities that flower from it were identifiable with the English and not with ourselves.

What relief this provided us. What a pleasure that we could admit how much we enjoyed class and status in others, because we never had to confront it - least in our own television programmming - with our own lives.

I am convinced that much of this split personality was at play in our rejection of the lavishly mounted production of "Beacon Hill" with which CBS sought to entice us in the fall of 1975.

We wouldn't buy. Granted, the acting was overblown and the writing and direction chose, time after time, the obvious over the implied. But what seemed to be at the heart of our rejection of "Beacon Hill" was that it sought to lure our attention to a premise that makes us uncomfortable - namely, that as a nation and as a people, we do put on airs.

My hunch is that the network that devotes the greatest and most imaginative coverage to Queen Elizabeth's Jubilee celebration on June 6 will be the network that attracts the largest number of viewers. They will also be lured by the regal unfolding of the ultimate in class.

In our country today, the playing of "Ruffles and Flourishes" and "Hail to the Chief" has been dispensed with. But I often wonder whether the large number of viewers attracted to the "Eleanor and Franklin" series are not fascinated withthe patrician values we associate with two people who represented the closest approximation to the responsibilities of class that American politics has offered in this century.

We live in an age when television, newspapers and magazine have done their best - or their worst - in makingnearly everyone famous for varying amounts of time. My instinct is that most of us know that this instantaneous fame carries with it a certain leveling of the idea of class, and promotes a sense of commonness.

We are not comfortable with this, although we seem to devour its manifestations every chance we get: Farrah Fawcett-Majors posters, Fonz T-Shirts and the like. That is why, in our fantasy lives, we have so much enjoyed those who have displayed, each according to his or her class, certain constant values both upstairs and downstairs at 165 Eaton Place.

And that is why we will miss this series. It allowed us, each in his or her own way, to admit the pleasures we found in a world where the idea of class mattered; and to admit how much we yearn after it in a society where we pretend it does not exist.