Seen at Cowling & Wilcox, Shoreditch, which admittedly is a chain... but full marks to them for wit.
London Art, near Finchley Road tube station, is my current local, and very helpful they are. Open 11-5 on Sundays, too.
Back at the flat, my nearest source of art supplies, albeit a patchy one, is Fish and Cook, a long-established stationers, open 9-6, Monday to Saturday.
On the Clerkenwell Road, Stevensons packs an unbelievable amount of materials into a "deceptively small" shop. (For some reason it's not on the map.)
Cornelissen, near the British Museum, is a lovely old-fashioned shop.
Others, for you who live in other areas of London, are on the interactive map (here), and most have websites for online shopping - eg Jacksons.