Calling my Turdite comrades in the UK...

In a few weeks I'm going to be in the Cambridge area and was wondering on recommendations for a good coin shop? I'd like to get some souvenirs while I'm there of the silver variety... Any thoughts? I'll also probably be in London for a day toward the end of my trip with some free time on my hands... I've already seen most of the touristy sites in both cities, so more interested in visiting a coin shop or two this time around.