Main London marinas
are: St Katharine's, South Dock
, Limehouse Basin. Otherwise the alternatives include The River Medway and the Essex/Suffolk rivers, all within easy train ride of London.
Your best bet would be to buy a copy of The East Coast
Pilot East Coast Pilot
Also a copy of Reed's Almanac is invaluble, it is stuffed with information and covers coastal Europe
from the tip of Jutland to Gibraltar