3 days in London, no kids and my 40th birthday, where to shop?

[Themasterandmargaritas]

This is a once in a 10 year opportunity to shop on my own in the UK, where shall I go?

I need summer work clothes but more importantly I need a party frock.

I like Black Swan's suggestions, plus would also recommend Fenwicks which actually has a pretty good range (including some quite boutiquey brands on the top floor) nice lingerie section and a lovely 'powder room'.