The Civil Aeronautics Board yesterday approved a $256 roundtrip standby air fare between New York and London proposed by six airlines to compete with the no-reservations no-frills low-fare service of Laker Airways.

But it turned down the same fare for a "budget" service the carriers had proposed to offer to travelers who would be willing to pick the week in which they wanted to travel, allowing the carrier to pick the day, and would pay for the ticker three weeks in advance.

At the same time, the board said it would accept a higher "budget" fare from the airlines that reflected the increased administrative costs of the plan, such as the cost of the reservations, and also took into account the plan's more attractive features compared with the charter alternatives. The board instructed its staff to come up with a fare along the lines it suggested.

The board also turned down a third proposal by the carriers to lower the price of the off-peak advance purchase excursion (APEX) ticket between the two cities from $350 to $299. An APEX ticket has to be purchased.

The actions by the agency were tentative, subject to final action after the CAB staff prepares a formal order.

The proposals were submitted to the board as a package by the six members of the International Air Transport Association - including Pan American, World Airways and Trans World Airlines - which compete with Laker on the heavily-traveled route.

The board's decision undoubtedly will have a disruptive effecton the airlines and many passengers since Pan Am and TWA have been advertising heavily the availability of the proposed fares and in fact have sold out their allotment of "budget" seats for at least two weeks, beginning Sept. 15.

It is unlikely that the fares, in any case, could go into effect on Sept. 15 since the board's decision must be submitted to the President for approval. In a related development, the British Civil Aviation Authority yesterday approved the IATA carriers' proposed fares, but effective on Sept. 26, the date the new Laker service -at $236 round trip begins.

Under the IATA agreement, a total of 2,900 seats would be available each week in each direction, with Pan Am, TWA and British Airways each getting 700 seats. Air India getting 350 seats, Iran Air getting 250 seats, and El Al getting 200 seats.

The CAB had made its approval of Laker's service contingent on British acceptance of fare proposals by the IATA carriers that were competitively responsive to the Laker service. Yesterday in a three-hour open meeting, board members expressed the same concerns about the fares raised by the charter carriers, the Justice Department and the CAB's own Office of Consumer Advocate. They all argued that the fares might not be economically justified and could drive the charter carriers out of business, leaving the IATA carriers free later to raise fares with no viable competitive threat.