At the V&A, although most of its collection is currently in storage while Room 40 is under renovation, I managed to view a few pieces crafted by the incomparable Philip Treacy -- part of the much-lauded 2009 Hats exhibition (thankfully documented for posterity in this fabulous book)
And Atelier Millinery is a wonderful little haven just off Carnaby Street, a stone's throw from Liberty. Though a tiny space, it is full of wonderful sinamays, embellishments, and other millinery supplies. But Atelier is also chock full of hats for immediate purchase. Tina Giuntini, one of Atelier's instructors, was helping a pair of costumers seeking the perfect period toppers for a movie currently shooting near London when I arrived. She's an absolute font of information. Whether you're looking for help finding the perfect feather or crafting an amazing pillbox, she's the girl!