You need an 'unlocked' phone, i.e. not locked to work on a single UK network. Phones in the UK are heavily discounted in order to get your subscription business and often the provider will ensure that the phone they supply you won't work on anyone elses network (for example you take out a contract with Orange, and they ensure T-mobile sim cards won't be useable in it).
To find out whether your phone is locked, take your sim card out and put someone elses in (someone else who is with a different provider to you). If the phone works, it's unlocked and will work in Turkey with a Turkish sim card, if it doesn't it will not.
If it doesn't you have thhe option to get it unlocked for a small fee, there is always a man at your local sunday market/car boot sale who will do the job for you.
One last thing - phones have to be "dual-band" to work in Turkey - most are but you can check which ones are at
www.orange.comScunner