Assuming that you're heading to Dover, Sainsbury's have a travel money shop in the West Park Farm Retail Park at Folkestone, the exchange rate advertised is £1 = €1.115.
Wow, the best Travelex Pre-Booked Exchange for Pound to Euro is €1.11 and the current spot price is €1.14 , wonder why they are giving away Euro's at €1.25 to the £.