The battle for air passenger reservations on the route intensified when Virgin Atlantic airline cut its one-way fare from 129 pounds, or $152. The new fare will be offered until March 31, said a Virgin Atlantic spokesman. Pan American World Airways and TWA said they have a $378 round-trip fare. The People Express fare from Newark, N.J., to London is $159 one way. A TWA spokesman said: "Pending other developments in the marketplace, TWA does not plan any immediate response."