The problem could be buying the ticket in Paris on line if you have a credit card with a UK billing address..
I came across this piece of nonsense the other day on ba.com when trying to buy a ticket ex LHR using a debit card with a Spanish billing address. The 'country' field is preset to United Kingdom, there is no dropdown box and you cannot overwrite it. I fail to understand the logic of this and in fact it may well contravene the EU legislation stating that a ticket available on one EU country must be available in other EU countries. I did think of writing to BA to ask why but I suspect it will be a waste of time and just gives me another reason not to use them.