Even as a traveler with little know how one should know the airport & corner exchange shop are horrible. It is not uncommon to see a 20% or even 30% spread between some of the fairly common currency pairs. Not sure how many locals use those services though as the post office there offers reasonably tight spreads for fairly small cash exchange.
At £400 their rate is €1.0886. It goes to €1.0914 @ £500 & €1.0931 @ £1,000.
http://www.postoffice.co.uk/foreign-currencyThat's around a 20% difference between their rates and those scammy rates offered at airports.
I guess I understand absurd rates if you are cashing in rarely used Turkish Lira or some exotic far away currency, but if someone else is profitable at 20% difference for the same exact thing, that's an atrocious rate.
To put that 20% craziness in perspective, after Brexit vote the currency shift was about 10%.
https://www.theguardian.com/business/2016/jun/23/british-pound-given-boost-by-projected-remain-win-in-eu-referendumSo you'd need a couple more unforeseen and/or heavily bet against black swans like Brexit to make that exchange rate reasonable.
according to Oanda
https://www.oanda.com/currency/converter/the spot rate for £ to € is €1.11460.
Post Office spread
1.1146-1.0914=0.0232
0.0232/1.1146= 2.08% spread
exchange shop spread
1.1146-0.88=0.2346
0.2346/1.1146=21.04% spread
relative spread
21.04/2.08= charging 10.115X the cost for the exchange service as one can get as a walk off the street service at the post office
even a Travelex card looks like an extreme deal compared to those terrible currency exchange shops.
https://www.travelex.co.uk/currency/about-currency/euros