My understanding was that they control you until you pass the holding point for the runway. When you call "ready for departure" you are at the hold, and still under their control - so if they tell you to hold position, you are supposed to do just that.
Once they say "G-CD, surface wind xxx, take off your discretion" then you're in control. They also have the option of giving you traffic information and letting you make up your mind - e.g. "Cessna on 8 mile final, report lining up", to which you would reply "lining up" or "holding"
edit: FWIW, CAP413 says that "G-CD hold position" is an acceptable reply to "G-CD ready for departure"
