Overstress, it is exactly as you say. They are strongly suggesting that besides the legal minimum fuel you have to carry by law (ie the conditions you mention in your post) you should arrive in the London TMA with enough extra fuel to hold for at least 20 minutes before beginning the approach. Not a legal requirement, just an understated British way of saying it might be a Good Idea.
Also take note of that little paragraph which says that "no delay expected" might not mean what it says. Even if you are advised "no delay" you might still go around the hold for up to 20 minutes before beginning the approach. Thus the CAA rewrites the English language.
As far as I can remember, the wording is the same as the previous AIC - these things just have to be reissued from time to time to remain current.
Hope that helps.