I worked for Travelex and they are no longer at Gatwick, they are at select Tesco, Asda and Sainsbury stores. However, as a former employee I wouldn't use them as their rates are bloody pitiful in store. However, use them online via Tesco.com and you will get a better rate and over £500 will be free next day delivery (do it tomorrow before noon) or you can collect in store at the ordered rate for free no matter how much you order as soon as the order is made, so you can order, have a cuppa and it'll be there for you to pick up. If you don't want to risk that and want to get a decent rate, but, can only get into the local supermarket branch as they are open until 8pm most places and not into town, phone up all the travel bureaus in your town centre, find out the best rate and go to the supermarket Travelex and tell them that rate, Travelex do a Pricematch (they don't advertise this very well) with the best rate within 3 miles, so you'll get that rate instead of their advertised one, do not tell them you called by phone, tell them you physically went into the branch of Ramsdens or whoever. Can you see why I left, I felt their practices were sneaky and cruddy wages. Personally, I wouldn't use them, I got my currency from Ramsdens as it was a much better rate.
Whatever you do, do not get your currency at the airport! The rates are diabolical, I still cannot believe it when I see people doing it.