There have been wildly inflated purchases of movies at the Sundance Festival in the past, but none so extravagantly excessive as the $8-million buy of this quirky but curiously irritating British period comedy in which two boys with family problems join their dubious talents to make a sequel to Sylvester Stallone’s “First Blood.” The idea behind “Son of Rambow” sounds weird on the surface, but it’s made worse by treatment from writer-director Garth Jennings that accentuates the preciousness of the premise rather than muting it. Rather than winding up charming, the picture comes across as strange and vaguely off-putting.

Little Will Proudfoot (Bill Milner) is a shy, sheltered lad who recently lost his father to a heart attack and is being raised by his widowed mother (Jessica Stevenson), a member of the Brethren, a stern religious sect that lives a rigorous commune-like life. In school he’s accidentally thrown together one day with Lee Carter (Will Poulter), a trouble-making bully who’s been left in the care of his heedless older brother while their mother goes off on overseas trips; and Carter seizes upon the opportunity to con his wimpy classmate into becoming his lackey in making a home video version of Stallone’s movie. Will, who’s never been exposed to film or television before, effectively becomes the star of Carter’s project, happily engaging in all sorts of dangerous stunts in the process. Of course, his devotion to his new avocation will cause family turmoil, especially when an officious member of the Brethren (Neil Dudgeon), a self-appointed father (and husband) figure, perceives that he’s slacking off.

It’s easy to see what Jennings and his producer-partner Nick Goldsmith were after here: a nostalgic dramedy in which two polar-opposite tykes become unlikely chums who support one another through difficult transitions, spruced up with visually enlivening touches courtesy of Will’s colorfully imaginative drawings (sometimes given full rein in brief animated sequences) and shots from Lee’s video, and with enough twists on convention to give it an appealingly peculiar tone. But the picture’s oddity turns out to be a little creepy instead of engaging. For one thing, the boys just aren’t very likable. As played by the rail-thin, recessive Milner, Will’s rather a pathetic dweeb, and Lee, played ferociously by Poulter, is simply too nasty to garner the sympathy we’re meant to have for him at the end. The entire Brethren subplot comes off as too dark and forbidding (at times we seem to have wandered into M. Night Shyamalan’s “Village”), as does the stuff about Carter’s brother, whose conversion to niceness toward the close lacks any dramatic conviction. The slapstick bits involved in the video shoot are simply lame (most notably a bit involving a kite). And a final sequence in which Carter’s terrible video gets a warm reception from a crowd assembled at the local cinema to watch Barbra Streisand’s “Yentl” (the year’s 1983, you see) might work if it were played as a parody of such sloppily sentimental finales, complete with a crowd telling us how we’re supposed to feel, but unfortunately seems to be intended to be really uplifting.

All of that pales, though, beside the script’s worst invention—the character of an exotic French exchange student named Didier (played, or rather overplayed, by Jules Sitruk), who becomes the toast of the campus and, with Will’s help, briefly takes over Lee’s film (thus becoming the catalyst for the inevitable breach between the two friends). Set aside the implausibility of so peculiar a fellow being warmly embraced by a bunch of British kids—it’s far more probable that he would have been treated like a pariah than a messiah. The fact is that the whole thread isn’t particularly amusing, and a turn at the close, revealing that Didier too is an outsider among his fellow expatriates, comes out of the blue—(it’s something that’s not even suggested during the earlier footage).

“Son of Rambow” has a garish, flamboyant look (this is the same pair responsible, after all, for “A Hitchhiker’s Guide to the Galaxy”) and a score (by Joby Talbot) that’s equally indulgent. They fit a movie that tries hard to be different, only to prove that different doesn’t necessarily mean better, or even good.