The mantra for the long list of producers seeking berths for forthcoming West End musicals: Book early. But that’s easier said than done, given the current state of affairs in London theater.

While Broadway has 25 shows running, London has 37. And with attendance up over the prior year’s record tallies, most of London’s theaters are either doing good-enough business for incumbent producers to feel safe for the foreseeable future, are already pre-booked with product, or both.

The result? A major logjam.

That’s not good news for such U.S. product as “Million Dollar Quartet” and “Rock of Ages,” both seeking West End homes.

They also have to compete with homegrown fare including the forthcoming tuner based on Jacques Demy’s movie “The Umbrellas of Cherbourg.” The musical, adapted and directed by Emma Rice (“Brief Encounter”), has a pre-West End run in February at regional house the Curve in Leicester.

Earlier than that comes “Lend Me a Tenor — The Musical.” Based on Ken Ludwig’s enduring hit with book and lyrics by Peter Sham and music by Brad Carroll, the show opened for a pre-West End engagement at regional theater Plymouth Theater Royal on Sept. 24.

Then there’s “Five Guys Named Moe.” Paulette Randall helmed the hit tuner 20 years ago at outer London house Theater Royal Stratford East, andthe show is back there again, after drawing great notices at this year’s Edinburgh Festival. It certainly hasn’t harmed the musical that one of its cast (and its book writer) is Clarke Peters, now a major name thanks to his work in HBO’s “The Wire” and “Treme.”

And these are by no means the only tuners in contention for London stints.

Sheffield’s dynamic Crucible theater is producing a much-fancied revival of the Brit classic “Me and My Girl” for Christmas, and Anna Mackmin’s production is said to be aiming for a London run.

The long-planned English version of “Rebecca,” the Austrian tuner based on Daphne du Maurier’s novel and helmed by Francesca Zambello, is also waiting to land.

Offers are already out to cast members, and its producers’ sights are believed to be set on the Palace Theater, currently home to the Broadway-bound “Priscilla, Queen of the Desert.”

Others are also keen on that venue. Not least is a potential revival of Jerry Herman’s “Mame,” unseen in London since Ginger Rogers played the title role in 1969. But uncertainty prevails at that address, as it’s one of the Really Useful Group’s theaters up for sale. Who can tell which way a new owner may jump?

Meanwhile, producers who were eyeing the Aldwych, home to “Dirty Dancing” since September 2006, may have to think again since the recent dip in that show’s fortunes has been reversed thanks to the arrival of a new cast and advertising images that have resecured its tenure.

As to the more immediate future, the Royal Court is on the brink of announcing a transfer of Dominic Cooke’s meticulous, splendidly acted production of “Clybourne Park” by U.S. scribe Bruce Norris.

A balancing act of caustic social satire and riotous comedy, the production opened Sept. 2 to ecstatic notices and is, without doubt, London’s newest hit.

Contradicting the theory that theater critics no longer have impact in the Internet era, the raves induced the Court’s biggest-ever single sales day and turned a slow pre-opening advance sale into an SRO run.

Indications point to “Clybourne” moving in January to Cameron Mackintosh’s 750-seat Wyndham’s theater. But insiders whisper that the other venue under consideration is the Theater Royal Haymarket. That’s home to the Menier Chocolate Factory transfer of “Sweet Charity.”

In a town where no one publishes individual box office figures, it’s hard to get the true lowdown on business. However, word is that “Charity” is doing OK but not boffo biz.

Producers of the biggest hits, meanwhile, are unsurprisingly happy to announce success.

Take Kim Poster and Sonia Friedman’s revival of “All My Sons.” Howard Davies’ highly lauded production — not to be confused with the entirely separate 2008 Gotham revival helmed by Simon McBurney — opened at the Apollo in May, recouped in five weeks and, as Poster told Variety, recently took the highest-ever weekly gross in the Apollo Theater’s history.

“War Horse,” too, continues to break records — good news for its March arrival at Lincoln Center. Following its two SRO limited runs at the National Theater in 2007 and 2008, it transferred to the New London in March 2009 and recouped in 12 weeks. Booked till February, it broke its own record for the highest-ever weekly wrap recently.

Those last two statistics are significant given that early September is traditionally one of the toughest in the London calendar, with the exit of summer tourists and family business close to zero as kids return to school.

They’re a reflection of the overall figures for the year so far collated by the Society of London Theaters (SOLT). From January to March, attendance was up 7% up on the previous record-breaking year. The second quarter then dipped 7% due to recessionary pressures, limited travel due to the volcanic ash cloud, and the World Cup.

But there has been a bounceback. From July to the start of September, attendance was up 4%. The overall total is already up 1%, with the fall-winter figures still to come.

Even Paul James, commercial manager of SOLT, admits to a degree of surprise within the organization at how robust the figures have been.

With that kind of business, incoming producers may find themselves in a long queue before legit houses become available.