I always use the post office for convienience.Ten of us went to Lanzarote on a stag do in October last year.This included someone who works for Barclays in the Channel Islands,who told us that unless you are getting about £5k worth of currency the difference is at most is £20.I compared rates with my mate who went to a B de C at Gatwick and another who got his over there and on £500 of currency the difference was £2.50!Use wherever is most convienient to you