.Dear Jan,  I'm afraid that you could be out of luck,  the only shell shop
in London was Eaton's which was located at 30 Neal Street Covent Gardens,
but I suspect that the high rents in that area, forced the owner to close.
There is no specialist shell book shop, but you will find Foyle's well worth
a visit.  If you ever make it to Northern Ireland do come and see me, you
would be most welcome.
Kindest Regards  Stanley Francis.