NEW YORK — Jason Priestley, former star of TV’s “Beverly Hills, 90210,” has joined the London production of Warren Leight’s Tony-winning play “Side Man.”Show begins performances at the Apollo Theater in the West End on Feb. 17. Most of the play’s original Gotham cast members will be aboard, including Edie Falco, Emmy winner for “The Sopranos,” Tony winner Frank Wood, Kevin Geer, Michael Mastro, Angelica Torn and Jeff Binder. Production will be directed by Michael Mayer, who helmed the original Gotham Off Broadway and subsequent Broadway stands. Priestley, best known for his years on the hit Fox TV show, has recently been concentrating on directing. His first docu, “Barenaked in America,” preemed at the 1999 Toronto Film Festival.