Several answers to the above problem.
1: Just carry on paying your monthly direct debit but don't use the sim card (if you do this ensure you give them a months notice to end the contract when it's due).
2: You can pay the rest of your contract up front by closing your account with Vodafone UK & paying the balance by debit/credit card. If you do this make sure your iphone is completely synced with itunes as Vodafone may "zap" it thereby losing your contacts etc.
onto how to use the phone in Turkey:
3: Register the handset with Vodafone TR, the phone will not need to be "unlocked" as such, just the settings for the TR side of things to be changed & that's an automatic change on the iphone, you will be sent several text messages & "invited" to save the settings as default, these are for sms, mms, internet etc.
4: Getting the iphone unlocked in the UK is not quite so simple if it is currently locked to Vodafone, there are ways round it but you need to take it to a specialist phone shop, whatever you do don't waste your time paying for an unlocking "code" from EBay, most are simply a scam.
5: Getting the iphone unlocked in Turkey may be easier however I have no knowledge of this.
HTH
Mark.