"as was the fad for singing with a smile on the face"
Not a "fad" - it was a technique adapted by Bert Lloyd to produce a hard tone, which he found extremely difficult - as far as I know, Bert was the only singer to use it regularly in public, though I am aware that some singers used it while working privately on tones.
Bert claimed he picked up the technique from recording Eastern European singer - it was quite common for some singers from other cultures to use such techniques (singing with the hand over the ear being another centuries, probably millennia-old one).
Personally, while I find Ken Wilson's singing pleasant enough, for me, his somewhat thrusting approach which produces a gap after each line leads to a lack of flow, which I feel Frankie gives it.
Far from it being "hard", certainly compared to many of her songs, I find Frankie's lyrical approach produces the tenderness that the song needs.
Horses for courses, I suppose.
Jim Carroll