The Wonder Room at Selfridges.
Watches of Switzerland various branches
Wempe on Bond St
The Watch Gallery
Full dealer list is on the official Panerai site
Of the above my personal recommendation would be Selfridges and speak to Tim
Secondary / Grey market places are Grays market near Bond St tube and a couple of places on Bond St and the Royal and Burlington Arcades
"Going to London, what for mate, the Jubilee, the Olympics?"
"Nah, P-Day mate!!"