Westfield shopping centre isn't all that. The rents must be horrendously expensive as the shops are tiny. Better off sticking to West End as you'll see more rather than be inside an anonymous mall.

One tip for being in London - take the time to look up now and again as some of the buildings are splendid and you don't normally get to see them if all you are doing is looking at shops/pavement/buskers/muggers/immigrants etc