I frequently book tickets like this without issue although as a UK registered member my taxes are in pounds regardless of whether my booking starts in the UK or overseas.

Do you get the correct taxes if you book CDG-LHR?

If you do then phone up and book, tell them the website is showing incorrect taxes and they should waive any booking fees due to the website issue.

If you don't see the correct taxes on CDG-LHR then do you qualify for RFS? You need to have made some sort of transaction within the last year. If you have not, don't worry we can help you with ideas for that!