Lawrence & Co – Based in Brighton Lawrence have a great shop and an efficient on-line business – they sell almost everything you could need
Cult Pens – my go to place for pens and markers of any kind. Check it out!
Jacksons Art – great for general art supplies. Like Lawrence they stock the wonderful Golden Liquid acrylics
Art from the Heart based in Harrogate this is a wonderful resource selling everything from Stencil Girl to Tim Holz and a fabulous range of stamps and mediums. Basically they stock a lot of lines that are hard to source outside of the US. They also run mixed media workshops.
Great Art – another general art supplies store good for printing
The following stores specialise in print making supplies
Hawthorne my go to place for print making. They sell every thing from their own presses to their own brand of oil based paint (recommended).
Intaglio – good selection especially cutting tools
Hand Printed – not used them yet myself but worth checking out