Quick post to mention another good materials source.
Falkiner’s (formally Shepherds Bookbinder Ltd) on Southampton Row (near Holborn) – specialist bookbinding supplies. Good paper in large quantities, thin leather of just about any description (including stingray, parrot fish, perch, and cane toad skins), and Useful Tools. They also sell good-quality lightfast pigment ink calligraphy pens, too.
They do leather scraps & offcuts at £3.50 for 100g, and they aren’t stingy about what goes into the offcut box either – I picked up some very nice pieces.