A truism at the heart of any affair or breakup is that inevitably one person makes the decision to move on and the other is shocked they weren’t “consulted.”

No matter how long the warning signs have been there, no matter how obvious it might seem to one party, even outsiders looking in, there’s always shock, wounding and a lack of that meaningless cliche “closure.”

That’s the secret sauce of writer-director William Nicholson’s biting but somewhat enervating “Hope Gap,” a very British chamber melodrama starring American Oscar winner Annette Bening and reliable British brooder Bill Nighy.

They’ve been together 29 years, Grace with her passion for poetry, dropping W.B. Yeats on old friends, “Nor public man, nor cheering crowds, A lonely impulse of delight,” history teacher Edward adding “wiki” touches to his lectures about Napoleon’s “Retreat from Moscow.”

She misses their adult son (Josh O’Connor), but when he’s summoned from the city, she starts in on him about his non-belief. He was raised Catholic, after all. Dad’s the peacekeeper, looking for some non-confrontational way of letting him off the hook.

Faith is like “love. You don’t tell love, you feel it.”

Her interrogation of Jamie, about his singlehood, his love life, has a kind of relentless quality. The kid says he’s “fine” and it’s “fine isn’t the same as HAPPY,” she says. WE’RE happy, aren’t we Edward?”

Her husband’s eyes-averting agreement is the entire marriage in a sentence.

“Yes. We’re fine.”

Three words give away the game. Edward, who is slapped in the middle of one “Why don’t we ever TALK, the way people do?” tussle, has had enough. He’s talked the son into showing up so he can break the news to him first.

She can’t hear him when he tells her, is taken aback with the key nugget in the declaration, “There’s someone else.” And damned if she’s going to accept this fait accompli. She wasn’t “consulted.”

What follows is a lovelorn son pulled in two directions by parents who are breaking apart. He doesn’t have to choose sides, but he needs to keep his contacts with his father limited in scope (no meeting the new woman) and pretty much secret. Dad is expecting Jamie to deliver this or that bit of bookkeeping business — divorce papers, “You can have the house,” etc.

Mom? She’s furious, as only Annette Bening can make her.

“He’s MURDERING a marriage. Marriages don’t bleed, but it’s still MURDER.”

Screenwriter (“Gladiator,” “Unbroken”) and sometime writer-director (“Firelight”) William Nicholson gets more good lines in than good scenes, here.

The casting makes for a little embrace of this stereotype (Nighy is the very picture of English introversion) and shattering of another. Bening, playing “British,” is more an American cliche — outspoken, angry, demanding answers and/or satisfaction.

The weakest link is O’Connor, kind of squishy in a squishy “Why can’t I make anybody love me?” role. Meeting with a couple who are both friends with him give the film a little variety from its three-hander intimacy (with lovely seaside settings, the “White Cliffs of Dover” included). But they don’t advance the plot or illuminate the battling parents.

Jamie is, therefor, the screenwriterly product of a dysfunctional marriage — unable to speak up (like his Dad), not satisfied enough to leave things be (like his Mother). He’s more his Dad, if one has to put him on the couch.

It’s always lovely to see Bening and Nighy, always a warm delight to set some of this tug of war on the pebbly beaches, rocky crags and chalky cliffs. Otherwise, it’s a “kitchen sink” drama, without many blowups, no big shocks and not a lot that sticks to the ribs after the credits have rolled.

MPAA Rating: PG-13 for some thematic elements and brief strong language

Cast: Annette Bening, Bill Nighy, Josh O’Connor

Credits: Written and directed by William Nicholson. A Screen Media release of a Roadside Attractions production.

Running time: 1:40