No replies Joe?
All the cockney Basenoters must be busy sewing pearly buttons onto their caps . Hope an uncouth northerner can help in their absence.
I'd suggest a stroll around Jermyn St, St James St and Old Bond St. They are all within 5/10 minutes of each other. You should be able to get around 6/7 places in a few hours. I don't know how available these are in the US but you could visit Trumper, Ormonde Jayne, Floris, Czech & Speake, D.R. Harris, Taylors and Truefitt & Hill. The details are in the London shopping guide here on BN.
It's quite some time since I was shopping around there (and that was in pre-scent-obsessed-days) but it's a nice part of London to browse around, particularly if you are looking for English gent style stuff - cufflinks, ties etc (I'll never be able to afford to even look at the tailored suits, but a few nice accessories go a long way!)
I don't know of anywhere in London that sells the really obscure scents (Anglia Perfumery, Cotswold Perfumery etc). I think the best source is The English Scent http://www.english-scent.de/index2.html which is, naturally, in Germany!