On my EBTA (2019) there was a woman who wanted to 'turn in' some £ she had for England and DCL wouldn't do it. Because I knew I would be back to the UK, I offered to change them into US Dollars for her. I used to always try to get foreign currency. I now have about $500 in Euros, pounds, Swiss Francs and Australian dollars in a ZipLock bag. In truth, though, on my last trip to Scotland in January of last year, I just used my ATM card (which had no ATM or foreign exchange fees) pretty much everywhere without issue. And you can go to a bank ATM and make a withdrawal - it's so easy. The ATM machines at most airports are run by Exchange Companies like Travelex, not banks. So not the best place to do it. A great article by Rick Steves below about foreign currency. Getting a credit card with NO FOREIGN TRANSACTION fees is a must. And an ATM card with NO INTERNATIONAL ATM WITHDRAWAL FEES. When my niece was going to spend 5 week in Italy, we found out that Wells Fargo charges $5 for every overseas withdrawals in addition to a foreign exchange fee, so we found a better card for her. If nothing else, check eBay. I know, but sometimes people sell left over currency there if you just want €20 to get you started. I wouldn't buy from overseas, though.
Rick Steve's Advice on Foreign Exchange