We just got back from London less than a week ago and I am already ready to go back! One of my favorite adventures in London was visiting the local Boot Sale. We had never experienced anything like this- a place where people would drive up and open the backs of their trunks to sell anything and everything. Similar to a garage sale, there was mostly second hand "bits" like clothes and household items. Some people also brought truck loads of fresh vegetables or flowers- and then of course we found some vendors with loads of eclectic vintage things.
If you have the opportunity to explore London, I highly recommend experiencing a boot sale. Check out TimeOut London for info on the best places to see!