British government yesterday gave Laker Airways the authority to offer travelers cut-rate reserved seats on its flights across the Altantic.

In another development, Laker asked the Federal Aviation Administration for an immediate exemption from its order grounding the DC10s to enable its planes to land and take off within the United States. In its petition, Laker argued that the FAA's month-long investigation of the design of the wing engine pylon assembly confirms that it meets certification criteria and that, in addition, Laker's aircraft were never subject to the "maintenance abuse" identified as the cause of cracks found in some DC10s. None of Laker's DC10 pylon assemblies had any cracks, the petition stated.

Whenever the FAA order grounding the planes is lifted, yesterday's British government action clears the way for Laker to offer reserved seats on its New York-London and Los Angeles-London routes at fares between 20 and 25 percent lower than its competitors.

In a hearing before Britain's Civil Aviation Authority last month, Sir Freddie Laker, ebullient head of upstart airline, had complained that the major transatlantic airlines were cutting into his low-cost, no-frills, no-reservations Skytrain business by offering low standby fares but that his operating license prohibited him from trying to lure their customers away with cheaper reserved seats.

The "Big Three" airlines were trying to "kill off" his Skytrain service and would raise their fares once they had Laker contended. "I'm tired of being the prey with my hands tied behind my back," he told the CAA.

Armed with his expanded authority, Laker plans to offer a reserved-seat roundtrip fare between New York and London of $599 during the summer, $165 less than the fare now charged by British Airways, Pan American World Airways, and Trans World Airlines. Laker's reserved-seat fare will drop to $499 off-season.

Laker plans to continue offering the public the same-day reservation Skytrain standard service - $299 roundtrip during the summer and $279 off-season - and will also add an excursion service allowing bookings 21 days in advance for a seven-day minimum stay requirement. The roundtrip excursion ticket will cost between $329 and $419 depending on the season.

As with the standby Skytrain service, the passenger has to pay for any extras, including meals. With the expanded services, a passenger will also have to pay a $50 fee to change a reservation.

Laker reasons that it costs the air line more to make a reservation for a traveler than it would to accomodate a standby passenger so the person who wants the reservation should pay the costs associated with it. Likewise, it cost the airline more to change a reservation, so the person who wants to change one should pay those costs.

The British government action actually put Laker Airway on the same procedural basis as other airlines serving the United Kingdom by removing the restrictions that had been placed on it operating license. The action allows Laker to alter services and fares as easily as do other airlines - by filing tariff amendments for approval with the aviation authority.

The license Laker held until now limited his scheduled operations solely to the one-class Skytrain service, and changes could be made only through an elaborate hearing procedure.

Laker will also have to file its new fares with the Civil Aeronautics Board but a CAB official said yesterday there would be no problem with approval.

Laker's Skytrain service served exclusively by DC10s, was shut down entirely with the FAA grounding order a month ago today. Even though Britain's CAA has restored the certificates of airworthiness of DC10 aircraft operating in and out of Britain, Laker's operations here cannot be resumed until the FAA acts.

In its petition to the FAA Laker noted that Laker's DC10s are equipped with the dual stall warning systems - and were required since 1972 by the CAA - that FAA Administrator Langhorne Bond has recently ordered installed in all DC10s operating here.