If the pound tanks and passes parity (1:1) you'll still have to take a hit to sell the physical notes, so you may just about break even. Over the counter exchange is always a kick in the crown jewels. Though, you could try some of the online services that offer better rates. But that'll involve sending cash in the post.
Are you planning to visit Europe in the next 6-12 months and can afford to keep the notes stashed away?
Short term our exit from the EU will affect the pound but Europe may also be adversely affected, so it may pay to hold long term.