One of the first things you pick up on your travels is how to minimise paying for your local currency. That means you make sure you get the best exchange rates by not using airport bureau de changes, not taking out cash on your credit card via the ATM and when using your credit card, making sure it's a reputable business where your details won't be cloned or sold on. There is always an element of risk with the latter but if you are a repeat customer and never allow your card out of your sight, you should be OK.
Another tip (highlighted at the BBC) again links back to your credit plastic. It really is a good idea to make sure any transaction are processed in local currency as rates are loaded in the seller's favour, sometimes by a considerable margin.
We have noticed in the past that some establishments do not even offer you the option but input the figures into sterling automatically. Do not accept this, void the transaction and repeat it in whatever the local currency is.
The best way is to simply advise them before you hand over your card and ensure they acknowledge your request. No chance of any misunderstandings there then.
According to the piece at the Beeb, it costs UK tourists about £500 million a year with marked up rates between 6% - 10% on each transaction. That's worth keeping in your own wallet or purse.