It's always a risk changing all your money at once. You will usually get a better rate in Turkey as they really really want your sterling. However, as Legless says, if you think the rate is good enough then go for it. We did exactly that when we were over last week - paid all our site fees in one go at a rate of 3ytl to the pound. It could prove to be a wrong move if the ytl continues to fall in value but at the end of the day we got what we thought was a good rate and went for it.
If anyone knows which way the YTL is going to go please let me know and I will make my fortune!
Stoop