Hustle/WMG

Whenever one of the gang appears on another TV show, they are undercover for the purposes of a con.

 * Mickey may or may not secretly be a vampire (depending on whether you count the actors in the pilot of Being Human (UK) as canon).
 * Danny secretly has a girlfriend attached to a piece of paving stone in his room (Doctor Who, "Love and Monsters").
 * Or here's an inversion: Jack Harkness was just pretending to be a sleazy home-remedy tycoon as part of an undercover mission for Torchwood.

Albert Stroller is a retired Napoleon Solo.

 * With the end of THRUSH and the Cold War he found himself bored, so he now uses his undercover agent skills as a grifter; this way he can still make a living hurting bad guys. His backstory as a fired shoe salesman who went to Vegas for con jobs is just a convenient cover to explain where he got his skills.

Stacie hasn't come back to the show because she's dead.

 * If the above theory is true, then she's been killed by Dexter. She was last seen in America...
 * In canon, Stacey stayed in Ameria. So she moved to Miami and went batshit insane.
 * She was killed in Paris France iirc.
 * So, general theory confirmed, details Jossed?
 * No contradiction. Stacie was last seen in America; after splitting up with Danny she meets Dexter and manipulates him for some unknown purpose. She flew to France in the season 2 finale of Dexter, and he followed her there and killed her.
 * The episode of The Mentalist where Jaime Murray was one of the victims takes place before she appears on Dexter as "Lila". Dexter really killed her; on The Mentalist she was just faking it.
 * Not dead, bronzed.
 * Disproved by the final episode.

Eddie is playing a really really long con on the gang.

 * Why do you think he puts up with them stealing from him all the time? How does he manage to run a bar with no other customers apart from them even if they ever paid for their drinks? Don't you think its convenient that he happens to mention that he collected World War II memorabilia when the gang were performing a Wartime themed con?

Hustle and Leverage take place in the same world
Starke said Nate's crew was the nastiest on the left side of the Atlantic, who has that title on the right side?

A cross-over between these two shows would probably redefine the word EPIC.
 * Keeping in line with the above theory stating that every appearance by the actor in another show is the character running another con, Jane from Coupling is redefining the "long con".
 * Richard Chamberlain has now appeared on both shows. WMG away!

Ash is the son of Gene Hunt from Life On Mars / Ashes to Ashes.

 * Hence the striking family resemblance.
 * So given the finale of Ashes to Ashes Series 3

Everytime Ash plays an upper classmen of noble blood, he plays it so well because he is..

 * The strut, the way he talks even the visage screams blue blood. That's because he is, Ash is the son of a lesser lord who impregnated his mother a working glass girl he was stringing along and dumped her when she got pregnant. That left Ash with the look of someone of standing and the smarts of someone born on the other side of the track needing to survive. And every smarmy, unpleasant Upperclass twit he plays? That his private jab at his waste of a father.
 * Also explains him trying to be a better father to Sean. Something his father never was.
 * Combined with the 'other appearances' theory, does this mean that he's pretending to be the Home Secretary in Spooks to pull a long con on MI 5 or is he really the Home Secretary undercover for a better knowledge of the world of crime?

The characters are sociopaths or at least have sociopathic tendencies
Multiple times we've seen them rob and cheat the innocent and probably poor without any empathy to what they're doing in order to get funds for their long con and they've all been known to lie constantly.
 * They're con artists. It's almost a given that that would be the case.