On line you may get some clues from the archive of Flight but it will be a lot of reading.
Bearing in mind that a lot of the MRTs were police, groups I researched police use of helicopters way back and there were 'interactions' between Westmorland police and winch equipped S51s at Anthorn in 1954 and numerous police forces in 1956 under the Civil Defence banner. None of those spoke of "SAR" but of course it may just have been a terminology ommission, especially with hoists that were restricted in ability. Perhaps the specific term grew out of the later civil Bristow Whirlwind operations?