The other slip in the area, is that of Greenwich Yacht Club, just upstream of Murphy's Wharf, Charlton. They got a fantastic, new, and very wide slipway, you can easily launch/recover x2 RIBs at once
. It was constructed when they were relocated about 600m by the Millenium Dome construction. I am not sure what the arrangements are for access by the public or whether or not they make a charge, they are all pretty friendly there so it might be worth asking. Especially as it was taxpayers money that built it!