Maniac, I was in a similar position to you and bought the Zed outright after 3 years, so essentially had it as an asset (a depreciating one, yes), but owed nothing on it.
When I traded it in for the SLK, the dealer put forward a "cash back" offer, so rather than the full value of the Zed going down as a deposit, I got a cheque back for a percentage of its overall trade-in value, and the overall trade-in amount was not that far away from the £18K you mention! Merc actually advised not using the full trade-in value as a deposit. Additionally there was a dealer contribution, and the top-line price of the car was reduced. It's all swings and roundabouts but the finances worked for me, and look favorable for the end of the agreement.
I just get the impression that BMW's are pretty crap when it comes to trade-in time. Both Audi and Merc seem to offer more attractive deals and hold their values better... maybe I'm just looking in the wrong places!
Good luck!