Putting Sparks in Marks

"When I was 17 london meant Oxford Street," sang Tracey Thorn of the pop group Everything But the Girl back in 1988. And to this day, this 2-km-long shopping Mecca is swarmed by young women with money in their pockets. At Arcadia Group's Topshop, where you can buy a T shirt that says "Not if you were the last boy on earth!" while watching the latest Westlife video on giant wall monitors, the line at the checkout counter is already a dozen deep. And it's only 11 o'clock on a Thursday.

But cross the street to Marks &...