The St Bride Foundation is the home of the St Bride Library, an incredible resource of printing history, in the form of books, printed ephemera, and tools from the trade. It also hosts many fantastic talks each year – check their website for details.
G.F. Smith's permanent showroom, in Central London. Visits are by appointments only — But I would definitely recommend making the visit. All of their stocks are beautifully archived, along with example publications, and a rotating series of paper installations in both the window & basement, courtesy of Made Thought.