It's now probably easier to buy petrol in France than in Blighty, there being a greater number of small unmanned stations attached to rural supermarket or simple 'automatic only' stations plonked around the place.
The problem with Caxton type cards is down to the type of card they are. In a real sense they are nothing more than pre-charged charge cards, rather like an Oystercard for use on TfL is here in London. The card reader ( or maybe more importantly the vendor) cannot say with absolute certainty if the funds are 'real' and might not at that very moment being used for something else. Coupled to the occiasional problem that an automatic pump often creates say an EUR 90 debit against, say an EUR 25 transaction, the EUR 65 difference often only being reconciled maybe some hours later.
I have all but given up with paper cash when abroad and here in the UK, other than in the pub and the odd coffee bar. 'No charge' credit and debit cards (readily available) are much easier.