THE IDEA THAT airline service between the United States and London could be disrupted by a dispute between governments sounded so absurd that not many people took it seriously.Yet that was the prospect right up to the last minute, when negotiators finally agreed the other day on the terms of a new air treaty between the two allies. The difficulty in reaching that agreement underlines the importance of airline revenues to national economies and the problems that exist in trying to rationalize what is, in anybody's terms, a chaotic situation.

It is probably true, as both American and British officials insist, that the final agreement was a good one for both governments. It leaves most of the airlines concerned unhappy but with an arrangement that they cannot validly denounce as unworkable or unfair. The British did settle for substantially less than they originally wanted. They had sought an agreement under which the air traffic would be divided equally between the carriers of the two countries, an arrangement that would have improved substantially their international balance of payments situation. They accepted one more flexible than that, but it opens up some new American cities to their airlines and curtails competition on some routes.

Whether the agreement is good for the consumers remains to be seen. The reduction in the number of trans-Atlantic flights should mean fewer empty seats. Bigger payloads, in turn, should result - in theory, at least - in lower fares. But we will have to see those lower fares before we can be sure about that. The way the price of international air travel is fixed - by agreement among the airlines - is not particularly conducive to cheap fares. Much more likely to help air travellers is the no-reservation, low-fare service that Laker Airways hopes to start between New York and London in the fall.

It seems likely that this agreement is only the first of many in which bilateral air rights between the United States and other countries are renegotiated. American international airlines have done so well economically, in comparison with their foreign competitors, that the interest in reducing competition and splitting passenger loads is great. It was vital that the negotiations with the British not set a precedent that other governments could use in demanding sharp limitations on the existing rights of American airlines. In this particular, it seems to us, the United States got about as good a treaty as it could have expected.