Forget Harrods, its a zoo.
Go to the TATE MODERN in Southwark (Jubilee line or go to waterloo and take a cab there(paddington connects to the bakerloo line or transfer to jubilee line at baker street). The building itself is fantastic, the exhibition spaces are interesting and the area around it is also v.cool. I would say you would need 2 hours to see it superficially. It does have a cafe, espresso bar and a very good set of shops.
You can also see the maligned milennium bridge from the terrace.
If you are very efficient you can also hit the london eye or something.
Shopping at duty free is much like the high street shops, gap, tie rack, wilson's leathergoods, virgin, books etc and a harrods shop. Not that interesting, its the global economy.
have fun and dress for rain.
lala