You could try looking over Chards website, they used to have a section on investment silver.
www.24carat.co.uk (I have found them very slow to respond to emails in the past).
You might also try a google search for pre-decimal British coins. Some of these were 50% silver, its possible to find coin dealers offloading piles of worm examples as bullion. (I think Cards used to sell 10KG bags of these).
(You need to be carefull on ebay - theres a lot of silver plated lead about)