You would be suprised to know how many embassies, borderguards let alone airport staff seem unaware or unwilling of the directive. Not all memberstates apply the directive correctly or incooperate the directive correctly. The UK visa and immigration laws violate the directive on Freedom of Movement in several areas for instance. These cases are being dragged out in (EU) court and the EU commission but it's wheels turn slow, very slow.
I would advice to get papers that show you are family (marriage papers are required, if possible legalized and with an official translation in a language that the official can read), ofcourse your passports and a statement by you that you as a EU (UK) national are taking your spouse to join you for a short stay in an other EU/EEA country (France).
You can print the directive in several languages here, read the English text and print article 1-7 in French and English:
http://eur-lex.europa.eu/legal-content/ ... 32004L0038
Article 2 and 3 show that the directive applies to you and your family. Article 5 covers right of entry. Article 6 covers short stay up to 3 months.
France, as part of Schengen, has a handbook "Operational instructions for the application of the Visa Code are further specified in the Handbook for the processing of visa applications and the modification of issued visas" for embassy staff that explains the rights of the directive further (borderguards should have a simular handbook I believe), you can find it here in PDF form:
http://ec.europa.eu/dgs/home-affairs/wh ... dex_en.htm
A section ( III ) is dedicated to EU/EEA visas/entry. As you can see you should be granted every facility to get a free visa with minimum hassle. If this is no the case, complain to the embassy, possible it's ministry in the country that controls the embassy and the EU (Home affairs). You will also see that you should be granted entry at the border if your spouse has no visa. Sadly the tricky part would be getting to the EU border.
You may also wish to contact the EU's support forum, especially if you do find stubborn people. See info and contact details:
http://europa.eu/youreurope/citizens/tr ... dex_en.htm
If you meet difficulties, insist on them/you contact a (senior) border official who knows the rules. You will find stories on this forum of airliners denying boarding, and either asking a local border guard (lol) or calling to the EU state with a borderguard that does not know the rules and who says that indeed a visa is required. If you are still denied boarding, try to gather any evidence that you were in your right. Evidence that shows you and your wife were together and attempted to travel and access an EU memberstate other then your own and therefor were entitled to access.
I hope you succeed but jsut check out the stickied threads and you will see that your rights are not always respected and various parties involved do not know or refuse to know (or pretend) how to apply the directive correctly. THis forum has a load of topics covering failure of proper application of EU rights at embassies, borderguards, ministries, checkin staff etc. Quite shocking indeed, you wouldn't expect this to be the case 10 years down the lane!!
Personally I'd share my experience with EU Home Affairs regardless of the outcome. The embassy for instance is in error. One complaint or story may not make a difference but if sufficient feedback comes in then I believe in time the various parties involved will see to proper application of this directive.
PS: if traveling to France it's the easiest if you depart from the UK. Atleast you will be at it's border then and be able to confront the border staff. Same applies the other way around. People have been denied boarding at EU mainland airports heading to the UK (German or French citizens with non EU family for instance) but gained access to the UK at the border. So if this plan of yours fails (I hope not!) but your wife can come to the UK, enter France via Dover-Calais.