Same with my Gym. Got them on a technicality though. You should check your Ts&Cs for this too. It says that you must give 2 months notice of leaving the Gym. It doesnt say that you have to prove you gave them the 2 months notice or that you have to pay for the time during the 2 months notice.
What you do is this. The day you join the Gym send them a letter telling them that you are leaving sometime in the next 5 years or so. Of course they'll get this letter and laugh, but you have given them more than enough notice that you are leaving. You are then perfectly entitled to cancel your DD if the bank let you.
In your case i would just put a back dated letter of cancelation hidden and out of the way on the desk and then just cancel. If they question you plead ignorance and that you left the letter on the desk months ago and its their problem. Sneaky and a bitbelow the waste, but then so is them asking you for 2payments when you wont even be there.