Hey Claire,
Well, what is the idea exactly behind the photoshoot? would the model made to look as a static mannequin or do you have in mind something to make her stand from the rest of the people? Do you already know the location and whether you could do the photoshooting without anyone disturbing ( talking from experience.
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I would suggest to use a red-haired model or at least a brunette with fair skin and more of gamine looks, not too striking features.
Black- white or black-grey or for a more interesting combination 2 tone purple, strippy top or jumper (u can easily find them at new look) over a white shirt. ( cotton or silk )
You can add a loosely- worn black tie.
Light grey, drain-pipe tight jeans and ankle length, high heeled boots in a striking color ( emerald, blue e.t.c )
Take a trip (since you're in London ) at Portobello for any original finds and you're bound to come up with more ideas.