There may be differences between debit card and credit card. I have always (for the last 30+ years) had a credit card (Mastercard) which I have used indifferently in France and UK (a french card of course). France is very card orientated these days, even for tiny sums (0.90€ for the parking, 2-3€ in the supermarket no problem) and no contact most of the time. There was a time a couple of decades ago when UK cards didn't have PIN numbers and wouldn't work over here (I have used my card to pay for petrol in the middle of the night for a brit more than once) but that is no longer the case and with no contact just about everywhere there shouldn't be a problem. That is credit cards. For those with cards that debit straight away (which do exist in France, I don't know for UK) going international is always going to be problematic.
It should be pointed out that broken down card readers and machine connections that don't work are everywhere as well, principally in petrol stations. If your card doesn't work there is a good chance that the machine is broke or just having a bit of a sulk. Try the machine next door, it will usually be fine (for those of you who need that sort of service in the middle of the night, hopefully not too odten on PBP).
I do not know if automatic pizza and baguette machines take cards. I would assume yes as it makes more sense but I have never used one. A roadside buvette, fruit stall would normally use cash. Markets these days are changing and quite a lot of stallholders seem to switch on to cards, it makes keeping the books easier and cuts the need for loose cash, but they will always take cash as well because their traditional clients are cash orientated