If you are a European store, and say you receive in EUR and charge in another currency it is highly disadvantageous to you. The payment processor usually charges a fee to convert the currency. Stripe for example charges 2% to convert (which is worse than the 1.5% interchange fee increase, and is is additive to it). You are significantly worse off in this situation.

Also if you are a British consumer simply pay in Euros, and let your card do the conversion for you - you typically get the interbank rate with most modern banks.