I'm not sure what the rate is at the moment, but we have always changed our money there.
If you are going to Borovets, one of the best rates we found was at the Bank in the bottom Gondola station. If memory serves me correct we were getting about 2.8 levs to the £1.00.
Currency Express deliver to your door if you need before you go and are giving 2.51 Levs/£1
Travelex do same rate but collect at airport but only certain ones
both have web site