You want the London heli-lanes chart: you're a bit short of time to get it mail order, but any pilot's shop will have one - around London at least. I'm not sure there is one at Denham, but there's the Pooley's shop at Elstree.
If your feeling brave and reckless and have a Skymap IIIc in your 206, you can find them marked on there. You might need to open them up in the menu (I can't remember), but your database needs to be right up to date for all operating altitudes etc.
You're best calling Heathrow NATS to let them know your callsign, routing etc before lifting, if you intend to use the routes.
Please - Stick to the standard operating altitudes and obey the ATCO's! the helilanes are a privilege and there's lots of people who don't like helo flying over their precious city!