The United States and England reached an air services agreement yesterday that will result immediately in lower fares to London from many U.S. cities, including Washington.

Some of the low fares, including a $283 roundtrip from Washington, take effect today. Both governments reacted swiftly yesterday to airline requests for approval of their proposals.

The agreement also cleared the way for the start-up of Braniff Airways' new service between Dallas and London, delayed since March 1 while the two governments hashed out their disagreements.

Braniff will start service today.

Although the British rejected several U.S. airlines' low-fare proposals, it was the Braniff plan that preciptated a diplomatic confrontation between the British government's rejection of some low fares the two nations when the U.S. refused to go along with the higher fares the British would permit. Two weeks of negotiations followed that U.S. move, ending yesterday in a new agreement.

When the negotiations opened, President Carter warned that the British action violated Bermuda II, the bilateral air agreement signed last summer. He threatened possible retaliation against British Caledonian Airways, which recently opened a Houston-London route, if the negotiations failed to produce a satisfactory resolution.

A second agreement worked out during the recent talks liberalizes the rules that apply to charter flights between the two countries, leaving capacity and fare issues to be set in the competitive market rather than by the governments.

"Taken together, these two agreements constitute a major step forward in implementing our key objective in the international aviation - to provide the traveling public with a wide choice of low fares . . ." President Carter said yesterday.

The new fares made possible by the agreement will let travelers fly to London on regularly scheduled airlines from 14 major cities for less than half the price of normal economy fares.

Those discounts had been available only on flights from New York. British Airways, Pan American World Airways, Trans World Airlines and other carriers flying between New York and London had come up with low fare packages in response to the Skytrain discount service started by Laker Airways last September.

Now, travelers from Anchorage, Atlanta, Boston, Chicago, Dallas, Detroit, Houston, Los Angeles, Miami, Philadelphia, San Francisco, Seattle and Washington also may take advantage of the standby and budget fares.

To take advantage of the new fares, standby passengers must appear at the airport at least three hours before departure time to receive a boarding pass if seats are available.

Passengers using the budget fare must purchase their tickets at least three weeks in advance of the week they wish to depart. The airline is to contact them at least 10 days in advance with confirmation of the exact flight and time of departure.

The number of budget and standby fare passengers is limited to a certain percentage of seats on each route for the major airlines.

The $283 roundtrip standby or budget fare between Washington and London compares with a $680 roundtrip regular economy fare. However, the airlines are expected to raise the discount fares during the summer months by as much as $50 roundtrip. Pan Am said it would raise the Washington-London fare to $326 during the peak travel period.

The agreement requires both sides to give "prompt and sympathetic" consideration to future low-fare proposals.

The charter agreement relaxes some restrictions, but does not go as far as U.S. officials woould have liked nor as far as the recent agreement the U.S. signed with the Netherlands. Under the Dutch agreement, the Dutch agreed to accept any charter flights from the United States that meet current and future U.S. charter rules. The Civil Aeronautics Board currently is considering removing almost all charter restrictions.

In contrast, the agreement with the British set a 21-day advance purchase period (reduced from 45 days last year), reduces the required group size from 40 to 20, and allows a charter operator to substitute up to 10 percent of the passengers originally signed up to 5 days prior to departure.

During a briefing at the State Department yesterday, Britain's chief negotiator, Patrick Shovelton, Deputy Secretary for Civil Aviation and Shipping, complained that the two governments had agreed on a more liberal charter agreement. Shovelton said the more liberal pact was "dashed from our lips" at the last minute when the U.S. government withdrew it after the U.S. charter carriers objected to it. "They obviously felt they couldn't stand the competition," he said.

The provision, proposed by the United States, would have allowed the scheduled carriers to use up to 15 percent of their seats for charter passengers, and would have allowed charter airlines to fill up 15 percent of the charter flights with passengers who sign on as late as five days before the flight departs.

James R. Atwood, Deputy Assistant Secretary of State for Transportation Affairs, who headed the U.S. delegation, said the proposal was put forward "in good faith" as a way to narrow the gap between scheduled and charter services and benefit consumers.

He noted, however, that unanimous support for the provision eroded when some government officials became concerned that "part-charter" competition was going to be so severe it would drive the charter airlines from the marketplace.

Although Atwood said he never spoke to Senate Chairman Howard W. Cannon (D-Nev.) about the provision.