Of all the “Mad Money,” “Poms” and “Book Club” trifles that Oscar winner Diane Keaton has made since stardom faded, “Arthur’s Whisky” might be the most trifling.

But this British nothing of a “fountain of youth” comedy manages to go down easily, despite or even thanks to its triviality.

Patricia Hodge, a mainstay of British TV (“All Creatures Great and Small,” the latest version), the singing sprite Lulu (“To Sir with Love”) and Keaton play three longtime friends who benefit from an elixir invented by Joan’s (Hodge) husband, who was promptly struck by lightning in his “Eureka!” moment.

How convenient. The entire screenplay’s a set of such conveniences.

We never learn how the three mismatched personalities met, never “get” the connection, for instance. It’s just there.

And when they drink this “whisky” that makes them young (Emse Lonsdale, Hannah Howland and Genevieve Gaunt give their all to impersonating Hodge, Lulu and Keaton in the bloom of youth), their “bucket list” of things they’d like to manage before they die, things they can survive that they’re young again, is inane and banal when it isn’t bathetic.

Hodge’s Joan doesn’t seem to miss her newly-dead husband (Ossian Perret) that much when he goes. There’s a reason for that. The American Linda (Keaton) has an ex she’s determined to get even with. Susan (Lulu) never married. Perhaps that’s something she can pull off once she’s a younger version of her cute self.

They stumble across the whisky and immediately set out to avail themselves of all the perks of youth — slang, clubbing, coffee shop hangs where they quaintly order Earl Grey tea, flirting and um, waxing — “Brazilian, Hollywood, Bikini or ‘landing strip?'”

Joan resolves to revisit an affair of her youth. Suze meets a handsome Venezuelan food truck owner (Adil Ray, not the most convincing “Venezuelan”). Linda wreaks havoc on her cheating ex’s new romance.

They travel, check off items from their “bucket list,” and manage all this even though this “whisky” has effects that wear off quickly.

The extent of the “message” to all this is “You’re never too old to become young.”

Cute bits include a vicar who can’t be bothered to get details right at funerals and the various eternal pick-up lines the ladies hear when they decide to “club” with the kids.

“Here I am. What’re your other two wishes?”

A bucket list trek to Vegas is an excuse to visit a drag revue that hosts a Boy George concert.

Lulu steals the picture — petty theft, in this case — and Gaunt gives us a fun, younger take on Keaton.

And that’s kind of how all this goes — “surprises” that aren’t, “bad news” that is almost expected and a tinkerer’s de-aging “whisky” which they never bother to investigate or try to replicate because who’d be bothered with a little thing like finding the formula?

Despite all that, the cast is pleasant and the Walton-on-Thames locations pretty. Of all the bad movies Keaton’s kept active with over the past 20 years, this may be the least of the lot. It’s certainly the least grating.

Rating: 16+, adult themes

Cast: Patricia Hodge, Lulu, Emse Lonsdale, Hannah Howland, Genevieve Gaunt, Boy George, Adil Ray, and Diane Keaton

Credits: Directed by Stephen Cookson, scripted by Alexis Zegerman. A Sky Original/Vertical release on Amazon Prime.

Running time: 1:34