I think they will give you a price that they expect for the property. It will affect if you are S/O. As for me, I'm annuity and all I need is to clear the redemption balance (easily said than done). I could sell it for more or below the market price. The figure they will give is what they want. Interesting topic. Ultimately whatever the balance is on your loan is what needs to be cleared. Get what you can but some are selling below what they are worth just to be rid of it at this stage.
To the OP, no you cant just hand back the keys. Go in and discuss with DCC and find out first hand what/if your options are. As you all know, there is no flexiblity with this scheme.