My general rule in Europe is to pay most items under £10 or 10 euro in cash, and anything above with cards, if they take them. Paying cash for small items is generally quicker (if there is a queue behind), but I like the convenience of not having to carry a lot of cash for settling larger items, and to have record to keep and compare. Also under UK consumer credit law, anything you buy or service you pay for over £100 anywhere in EEA is covered by credit card company (if they are based in EEA), which gives you peace of mind.

Just encountered this online!!!

I buy Christmas presents for England from amazon.co.uk, so I don't have to pay international postage. This year the site asked me for the currency of my credit card. The associated help screen said I would be charged in that currency with an option to change, but in fact the checkout showed GBP with an option to change to USD. I will be monitoring the bill carefully! (The conversion was well above the xe.com rate.)

Yes, Amazon UK uses DCC for non-sterling cards but it's usually easy to opt out of it and being billed in sterling.