Think it's a bit of a muchness. Do what ever you feel better doing. Don't think the exchange rate is considerably different. Personally i am changing here, just to be safe.
Personally I've changed up GBP100 which gives me 270 BGN, the rest I'll change in the resort. The exchange rate doesn't fluctuate too drastically so it's neither here nor there
Used to be they were really restrict with their currency going in and out of the country, but think you can take about £3000 before you have to declare now.
I changed £100 for both of us at Marks & Spencer (didn't have to order), got 280 Lev. I then took £30 for emergency and used the ATM's for the rest (until I split my card ), I think my bank charged £1.50 each time I used my card, but the exchange rate is apparently better over there anyway. I say apparently because my money still hasn't been taken off the card yet
If you use a Nationwide Flex Account card in foreign (including Bulgarian) ATMs you get the interbank rate and no transaction charges - I got a rate of 2.88 to 2.94BGN to the Pound last June in Varna/Dobrich.