I am going out on the 17th Jan and I will exchange in the resort with sterling ( making sure the notes are unmarked) because there is more confidence in the pound at the moment and the rate has altered by 10% in the last week and my bet is it will get better in the next week also.
Always a better rate when you exchange in Bulgaria. Best rate though is with a Nationwide Flex account card at ATMs in resort. Better rate and no charges.