If you dont mind a change-over sausage side, you can get a good deal with Lufthansa from Manchester. We have book them this year for the first time because the flight times are good and they fly more often than easyjet
We will fly from Berlin to Thessaloniki, I did this some times in the last three years and catch the evening train from Thessaloniki to Sofia. This year we booked lated and have to pay 90€ within baggage for february the train costed another 30€ retourn tiket, but we will visit Melnik and so it's the best choice for us.
If you want to go to Bansko you have to change in Blagoevgrad or you can find car transfers.