It depends on how you are getting your money. If you are using a 0% fee debit card (Visa backed) to withdraw at ATMs you will be charged the rate that applies at the time you are billed by the Turkish bank. This is usually the next day so the charge will reflect the rate set at US close of business today. If you had taken money out yesterdy you would have got 2.608 TL to the pound. Obviously I can't tell you today's exchange until tomorrow. Given today's increase in the value of £ against TL I would expect it to be higher - perhaps 2.62.
If you are exchanging cash rates vary from banks and PTT, to reputable change agents, to that nice man who calls out to tourists passing by on the street.