+1 Portobello Road market which is open on Saturdays, lots there plus records.
UK HIFI is 240V plus 20% VAT is levied on virtually everything except used goods.
Recent changes have abolished the tax-free tourist scheme.
I'd think about taking in a show, lots to choose from, Plays, Ballet, Opera, Rock whatever.
Try Rules Restaurant, Charing Cross, same steak and kidney pie recipe for hundreds of years and the Hung, Drawn and Quartered Pub, across the road from the Tower of London.