NEW YORK (AP) _ Ailing Broadway is counting on two quick shots in the arm from ''Les Miserables'' and ''Starlight Express,'' lavish British musicals that open in the next four days with a staggering $18 million in combined advance ticket sales.

''There's nothing wrong with Broadway that a couple of hits won't cure. That debatable bromide had an element of plausibility last week,'' Variety said Wednesday in its weekly analysis of Broadway box office receipts.

''Les Miserables,'' based on the Victor Hugo novel, opens Thursday night and ''Starlight Express,'' the Andrew Lloyd Webber roller skating musical, opens Sunday.

Each grossed over $430,000 during previews last week. ''Les Miz'' was a virtual sell-out and ''Starlight'' posted a gross of more than 90 percent capacity, the trade paper said. The two musicals alone boosted total ticket sales on Broadway by 12.6 percent.

''Obviously, these two shows, which each week will bring in over 25,000 people and nearly a million dollars worth of business to Broadway, have to have an impact,'' said George Wachtel, director of research for the League of American Theaters and Producers.

''Broadway gets so much more publicity when you have enormous hits like this,'' he said. ''As a result, more people read about Broadway and then show up at the theater. That's the bottom line.''

He compared the situation to the 1975-76 season, which produced ''A Chorus Line,'' now in its 12th year at the Shubert Theater.

''That show made people open their eyes and look at Broadway,'' Wachtel said. More people began going to the theater, and production activity increased, he said, with popular shows like ''Equus,'' ''Dracula,'' ''Da,'' ''Deathtrap'' and ''The Best Little Whorehouse in Texas.''

''With a hit Broadway show, there is definitely a ripple effect,'' Wachtel said. The average theatergoer spends more on theater-related items than on the theater itself and an average out-of-towner spends three times as much, he said, with the benefits going mostly to hotels, retaurants and stores.

In terms of production, Wachtel feels Broadway already is on a roll this season. Twenty-five shows have opened and 14 more are planned before the end of the season in May. In each of the last two seasons, only 33 productions arrived, Wachtel said.

Already ''Me and My Girl,'' ''Broadway Bound'' and Jackie Mason's ''The World According to Me 3/8'' have proved to be sizable hits for the 1986-87 season.

Seven new plays are scheduled to open before the end of May including works by established playwrights August Wilson, Harvey Fierstein and John Pielmeier. Also on tap are revivals of ''Pygmalion,'' starring Peter O'Toole, ''The Comedy of Errors,'' with the Flying Karamazov Brothers, and Noel Coward's ''Blithe Spirit'' with Richard Chamberlain, Blythe Danner and Geraldine Page.

Next season should also bring several blockbusters. Already scheduled are Andrew Lloyd Webber's version of ''The Phantom of the Opera,'' and ''Chess,'' written by Tim Rice, Lloyd Webber's former partner, and two members of the rock group ABBA. In addition there are such shows as ''Into the Woods,'' a new musical by Stephen Sondheim and James Lapine, and a revival of ''Cabaret,'' starring Joel Grey, that already has started a long national tour.

Still, many new shows have had a difficult time surviving without advance buildup or good word of mouth. Several producers have fought back with low prices for some seats, cheaper even than the half-price tickets available at the Times Square tickets booth. Even ''Les Miserables,'' which has a top ticket price of $47.50, has set aside 200 balcony tickets at $16 for students for Monday through Thursday performances.

''The Nerd,'' a comedy starring Mark Hamill, has balcony seats for $6 while ''Stardust,'' a musical revue, offers $10 tickets for all performances. ''Fences,'' August Wilson's drama about a black family, opening March 26 and starring James Earl Jones, also has a deliberate cheap seat policy.

''This play speaks to the heart,'' said Carole Shorenstein Hays, producer of ''Fences.'' ''I wanted to make sure we could offer a price scale that was accessible to the people we most wanted to reach. That's why we have a $10 ticket in the balcony on weeknights and $8 for matinees. To really survive, Broadway will have to bring back the people who can't afford the $40 ticket.''