Exactly where in London are you ?
Many folk from here launch at Putney . To the East , launching at Poplar Rowing Club ( almost immediately opposite Greenwich ) is possible , as is Bargehouse Lane in Woolwich at slack high tide . These later two depend on size of Rib , towing vehicle and possiblly experience of skipper .
Try the search function on RIBnet , also look at the Boatlaunch website and also the PLA website .
Seven metre tides , which can flow pretty fast should not be under-estimated but some famous sights to be seen. Feel free to come back and ask for more info