When the Whales Came or Dark of the Sun might not be generally known scores (though I love both), but their composers are hardly "non prolific" in my opinion--just look at the length of their filmographies at IMDB. Same with other composers already mentioned above.
I think Alan Price's THE WHALES OF AUGUST qualifies well.
Price had previously worked for Lindsay Anderson, providing a successful song score for O Lucky Man! and some incidental cues for the follow-up Britannia Hospital, but both director and composer seemed an unusual choice for THE WHALES OF AUGUST. Interesting to note that one of the main themes in WHALES is taken from Price's stage musical ANDY CAPP and works surprisingly well in a sea shanty arrangement.
There was a time in the 1980s when Mark Knopfler almost seemed like a one-man musical spokesperson for the British Isles, with scores for such films as Bill Forsyth's Scottish-set Local Hero and Comfort and Joy, and the Irish drama Cal, but seems largely to have abandoned film scoring to return to his day job.
John Carpenter didn't score too many films. Right about sixteen I would say. But some of the stuff he wrote are really great and music from films like Assault On Precinct 13 and Halloween is considered his best.
FLESH FOR FRANKENSTEIN-73- BLOOD FOR DRACULA-73-CLAUDIO GIZZI-Both fine scores and maybe more then any other example over the years after given us these 2 why did MR GIZZI leave film scoring? also did WHAT-73.