In my experience, online currency ordering via www.travelex.co.uk gives you the best rate (they also have a best rate guarantee). Order online and pick up at the airport. Just be wary that some debit card providers charge a 'cash advance' fee - however, Switch cards do not incur such a fee. My tip would be to order via www.baa.com/worldpoints and get the same great rate but also collect worldpoints (which you can use for frequent flyer miles or discounts). Remember to present your card when collecting the currency.
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