I have been frequently scammed by UK retailers who instead of debiting my credit card in Pounds sterling, carry out a currency conversion and debit it in my home currency. They claim that their conversion is advantageous for me as there is no commission on the exchange. That may be true, but they apply an extremely disadvantageous exchange rate.
When I catch them at it, I refuse to authorize the transaction.
As an example, taken from my credit card statement, a hotel tried to debit the extras I had purchased (35.65 GBP) in Swiss Francs and charged me 66.40 CHF. I refused to authorize the charge, so it was recredited to me. My own bank only charged me 65.00 CHF for the transaction (including 1.25% commission).
I don't know who is making money out of the scam - the UK banks, the retailers - and the amounts aren't huge, but in this case it came out to 2.15% of the price charged.
Remember you have the right to pay in pounds sterling in the UK. It's the only legal currency and retailers are obliged to accept them.