Melanie C makes sense. She's just won an award for JCS, and she's fine, and a bit different, in that. She was superb in Blood Brothers (as an refreshingly young and realistic antidote to the dodgy accents, dull portrayals and overly old characters often playing that role) and deserved her nominations for that too . Kimberley is inexplicable - unless she has another role coming up - though I suspect she would look better/slicker now after doing SCD.
More inexplicable is the lack of West End talent singing anything else much. You could easily find current performers who could sing most of the songs given to celebrity/US stars as well or better. He could even find people from his own TV shows who could sing many better. its just a bit odd - why you would fill a show with non West End people if you want to advertise your wares, and one or two big names would do to get the TV audience in? You would think he would want to show his achievements off at their best, rather than jump on the lets use a name bandwagon.