Lee, Liberty is a must; the le Labos are £80 for 50ml, and they stock Miller Harris there too (including the Liberty's exclusive Rose en Noir or whatever it's called).
The Prada exlusives are available in Harrods and Selfridges, £140. Harrods had three new ones last week - Narcisse, Violet and Tuberose, I think.
You might want to include Ormonde Jayne in your route, which is only a short distance from Fortnum & Masons, and also near Dover Street Market which is actually Comme des Garcons.
Take energy pills and wear very comfy shoes.