I am just starting out with leather. Last week I went to S C Hall in Partridge Green, West Sussex to buy some leather, I bought a natural veg tan side, tan coloured shoulders and some practice leather. They had a great selection (minimum order of 1 skin) and were so helpful. All different types of hides and skins at good prices. They do not sell online as such, but would be willing to send out a sample or two. The company was previously based in Bermondsey, London so is relatively new to Sussex.
Thought I would post this as the company doesn't appear easily in an internet search. NB I am not affiliated in any way, just a happy customer!