Leather comes by mail order from Le Prevo, who are excellent.
Bowls come from markets/Tintagel.
What you can get in London is:
Wool and linen - shops on Goldsworth Road. Take the tube to Goldsworth Road. Walk out of the exit. Look across the road. Just to the right of the Shepherds Bush Market entrance (as you look at it) there is Classic Textiles. Upstairs for wool, down for linen. There are other good shops up and down that bit of road also.
Handweavers Studio in North London. The only place to get weaving yarns. An amazing selection of wool, silk, linen etc etc etc. They have recently moved so check their new location and opening times on their website (search for Handweavers Studio).
Beads - you could try the obvious shop in Camden.