The Britannia International Hotel is at (near) Heron Quays on the Docklands Light Railway; also not far from Canary Wharf with access to the London Underground - see map below.
http://www.google.com/search?q=britannia%20international%20hotel%20london
Journey Planner - Heron Quays to Leicester Square 38 mins
http://www.tubeplanner.com/cgi-bin/planner/route.pl?start=Heron+Quays&end=Leicester+Square&via=&search_type=fastest&submit.x=70&submit.y=16&submit=Find+Route&search_type2=fastest
Journey Planner - Canary Wharf to Leicester Square 27 mins
http://www.tubeplanner.com/cgi-bin/planner/route.pl?start=Canary+Wharf&end=Leicester+Square&via=&search_type=fastest&submit.x=56&submit.y=18&submit=Find+Route&search_type2=fastest
Journey Planner - Heron Quays to Greenwich (Cutty Sark) 10 mins
http://www.tubeplanner.com/cgi-bin/planner/route.pl?start=Heron+Quays&end=Cutty+Sark+for+Maritime+Greenwich&via=&search_type=fastest&submit.x=62&submit.y=13&submit=Find+Route&search_type2=fastest
London Tube Map
http://www.bbc.co.uk/london/travel/downloads/tube_map.html
You can pick up a FREE pocket edition of the London Tube map at any Tube Stations.
http://www.webviews.co.uk/network/camera/england/london/tragalgarsquare/streamingcam.html
http://www.timeout.com/london/